If you work with DirSync and Exchange online, here's what you can do:
1. Create a CSV containing two columns- SamAccountName and ProxyAddresses (seperated in commas, in the format "smtp:address@domain")
2. On PowerShell import the active directory cmdlet by using "import-module active*"
3. Import CSV to a variable
$CSV=import-csv csvpath.csv
4. Following block will loop through users and addresses to add them
Foreach($User in $CSV)
{
Foreach($Address in $User.ProxyAddresses){Set-AdUser $User.SamAccountName -Add @{ProxyAddresses=$Address}}
}
5. Run a sync cycle to replicate into Office 365.
According to your other thread here you work with standalone Exchange online. If that's the case you can change the SamAccountName with mailbox alias and instead of "Set-AdUser" use "Set-Mailbox" on Exchange online shell.
Hope this helps. Feel free to let me know if you require any further assistance.
Maor Bracha