Migrating from Google Workspace with Multiple Domains

Hi hewoods23,

If I got correctly your doubts, what you're trying to do is quite easy. 

On one hand we have the domains and on the other hand the migration.

You can verify as many domains in O365 as you own, and set the default one that you prefer. In fact, you could verify your domains in O365 before the migration if you want and create your users with the desired one. For domain verification in O365 you only need to add a TXT record to your DNS. ( After that of course you'll need to perform further configurations for mail routing etc... But this steps are not necessary for domain verification and user creation ). 

- Add domains to your tenant: https://learn.microsoft.com/en-us/microsoft-365/admin/setup/add-domain?view=o365-worldwide

- Google Workspace migration prerequisites in Exchange Online: https://learn.microsoft.com/en-us/exchange/mailbox-migration/googleworkspace-migration-prerequisites?source=recommendations

- Automated Google Workspace migration: https://learn.microsoft.com/en-us/exchange/mailbox-migration/automated-migration-neweac?source=recommendations