Hi RobJBow - this is a big component for SharePoint hub sites, to be able to "give superpowers" to an existing modern team site or communication site. You will be able to run a PowerShell command (Register -SPOHubSites https://contoso.sharepoint.com/sites/Communications); where Communications is the existing site you will convert into a hub site. Once you do this, it becomes the hub that you can associate sites to, holds the cross-site nav, theme is inherited from, etc... you could create a new modern site and give it hub powers, or give it to an existing one - your choice/design.
We're working on hub sites Targeted Release very soon. The message center post went out last week as a heads up to Office 365 admins.
We’re glad to see you’re excited for the coming SharePoint hub sites. Everything we have to date is in this disclosure blog post (which also links to a webinar we gave around the same time): https://techcommunity.microsoft.com/t5/SharePoint-Blog/SharePoint-hub-sites-new-in-Office-365/ba-p/109547.
And soon we’ll have an updated “launch” blog to announce Targeted Release, new documentation for how to set up and configure them, plus a webinar to showcase it along with a broader intranet planning and strategy with Sue Hanley, Sam Marshal and myself on Mar.28.2018, “Intranet Strategy & Planning with SharePoint Hub Sites & Office 365”: https://collab365.community/events/intranet-sharepoint-hub-sites-office-365/.
Lots to come, and we’ll soon be very engaged with answering any open questions during an AMA on Mar.23.2018 within the SharePoint portion of the MS Tech Community: http://office.com/sharepoint/community
Thanks,
Mark