Is there a way to export custom properties with Export-SPOTaxonomy for a particular TermSet

You can use this below script for this. You dont need to Export / Import from one to another. This script will automatically copy Term Group with custom property. 

# Change the following to reflect your environments

# 1) Source Site
$sUrl = "https://<sourcedomain>.sharepoint.com/"
$sAdmin = "admin@<domain>.onmicrosoft.com"
$sPwd = "<password>"

# 2) Destination Site
$dUrl = "https://<destivation>.sharepoint.com/"                                                                                                  
$dAdmin = "admin@<domain>.onmicrosoft.com"
$dPwd = "<password>" 

# 3) What Term Group do you want to synchronize?
$sTermGroupName = "<Termgroupname>"

## Stop here
$lcid = "1033"

$sSecurePwd = ConvertTo-SecureString $sPwd -AsPlainText -Force
$dSecurePwd = ConvertTo-SecureString $dPwd -AsPlainText -Force

# these aren't required for the script to run, but help to develop
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"
# doh
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Taxonomy.dll"

# connect/authenticate to SharePoint Online and get ClientContext object.. 
$sCtx = New-Object Microsoft.SharePoint.Client.ClientContext($sUrl)
$sCredentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($sAdmin, $sSecurePwd)
$sCtx.Credentials = $sCredentials

$dCtx = New-Object Microsoft.SharePoint.Client.ClientContext($dUrl)
$dCredentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($dAdmin, $dSecurePwd)
$dCtx.Credentials = $dCredentials

$continue = 0

if (!$dCtx.ServerObjectIsNull.Value)
{
    Write-Host "Connected to DESTINATION SharePoint Online site: " $dCtx.Url "" -ForegroundColor Green
    $dTaxonomySession = [Microsoft.SharePoint.Client.Taxonomy.TaxonomySession]::GetTaxonomySession($dCtx)
    $dTaxonomySession.UpdateCache()
    $dCtx.Load($dTaxonomySession)
    $dCtx.ExecuteQuery()
    if (!$dTaxonomySession.ServerObjectIsNull)
    {
        Write-Host "Destination Taxonomy session initiated: " $dTaxonomySession.Path.Identity "" -ForegroundColor Green

        $dTermStore = $dTaxonomySession.GetDefaultSiteCollectionTermStore()
        $dCtx.Load($dTermStore)
        $dCtx.ExecuteQuery()

        if ($dTermStore.IsOnline) 
        {
            Write-Host "...Default Term Store connected:" $dTermStore.Id "" -ForegroundColor Green
            # $termStoreId will be the SspId in the taxonomy column configs

            $continue = 1
        }
    }
}

if (!$sCtx.ServerObjectIsNull.Value -and $continue -eq 1) 
{ 
    Write-Host "Connected to the SOURCE SharePoint Online site: " $sCtx.Url "" -ForegroundColor Green
    
    $sTaxonomySession = [Microsoft.SharePoint.Client.Taxonomy.TaxonomySession]::GetTaxonomySession($sCtx)
    $sTaxonomySession.UpdateCache()
    $sCtx.Load($sTaxonomySession)
    $sCtx.ExecuteQuery()
    if (!$sTaxonomySession.ServerObjectIsNull)
    {
        Write-Host "Source Taxonomy session initiated: " $sTaxonomySession.Path.Identity "" -ForegroundColor Green

        $sTermStore = $sTaxonomySession.GetDefaultSiteCollectionTermStore()
        $sCtx.Load($sTermStore)
        $sCtx.ExecuteQuery()

        if ($sTermStore.IsOnline) 
        {
            Write-Host "...Default Term Store connected:" $sTermStore.Id "" -ForegroundColor Green
            # $termStoreId will be the SspId in the taxonomy column configs
            
            $sCtx.Load($sTermStore.Groups)
            $sCtx.ExecuteQuery()

            foreach ($sTermGroup in $sTermStore.Groups)
            {
                if ($sTermGroup.Name -eq $sTermGroupName)
                {
                    Write-Host ".....Term Group loaded: " $sTermGroup.Name "-" $sTermGroup.Id "" -ForegroundColor Cyan
                    $sCtx.Load($sTermGroup.TermSets)
                    $sCtx.ExecuteQuery()

                    #create this group in the destination context
                    $newGroup = $dTermStore.CreateGroup($sTermGroup.Name, $sTermGroup.Id)
                    $dCtx.Load($newGroup)
                    $dCtx.ExecuteQuery()
                    Write-Host ".....Term Group copied to destination:" $newGroup.Name "" -ForegroundColor Magenta

                    foreach($sTermSet in $sTermGroup.TermSets)
                    {
                      try{
                        Write-Host ".......Term Set found: " $sTermSet.Name "-" $sTermSet.Id "" -ForegroundColor Cyan
                        $sCtx.Load($sTermSet.Terms)
                        $sCtx.Load($sTermSet.CustomSortOrder)
                        $sCtx.Load($sTermSet.Description)
                        $sCtx.ExecuteQuery()
                        }
                        catch
                            {}

                        #create new term set in destination context
                        $newTermSet = $newGroup.CreateTermSet($sTermSet.Name, $sTermSet.Id, $lcid)
                        $newTermSet.CustomSortOrder=$sTermSet.CustomSortOrder
                        $newTermSet.Description=$sTermSet.Description
                        $dCtx.Load($newTermSet)
                        $dCtx.ExecuteQuery()
                        Write-Host ".......Term Set copied to destination:" $newTermSet.Name "" -ForegroundColor Magenta
                             #create termstore Custom Properties copy 
                            foreach($TScp in $sTermSet.CustomProperties.GetEnumerator()){
                            try{
                            $newTermSet.SetCustomProperty($TScp.Key,$TScp.Value)
                            $dCtx.Load($newTermSet)
                            $dCtx.ExecuteQuery()
                            }
                            catch
                            {}
                           
                            }

                        foreach($sTerm in $sTermSet.Terms)
                        {
                            Write-Host ".........Term found: " $sTerm.Name "-" $sTerm.Id $sTerm.CustomProperties "" -ForegroundColor Cyan
                            
                            $sCtx.Load($sTerm.Labels)
                            $sCtx.ExecuteQuery()
                            #create new term in destination context
                            $newTerm = $newTermSet.CreateTerm($sTerm.Name, $lcid, $sTerm.Id)
                            $newterm.SetDescription($sTerm.Description,$lcid)
                            #  $newterm.CreateLabel($sTerm.Labels)
                            $dCtx.Load($newTerm)
                            $dCtx.ExecuteQuery()
                              #create term Custom Properties  
                            foreach($cp in $sTerm.CustomProperties.GetEnumerator()){
                            try{
                            $newTerm.SetCustomProperty($cp.Key,$cp.Value)
                            $dCtx.Load($newTerm)
                            $dCtx.ExecuteQuery()
                            }
                            catch
                            {}
                           
                            }
                             foreach($lable in $sTerm.Labels){
                            try{
                            $newterm.CreateLabel($lable.Value,$lcid,$lable.IsDefaultForLanguage)
                            $dCtx.Load($newTerm)
                            $dCtx.ExecuteQuery()
                            }
                            catch
                            {}
                           
                            }
                            Write-Host ".........Term copied to destination:" $newTerm.Name "" -ForegroundColor Magenta
                        }
                    }
                }
            }
        }
    }
}