Adding Domain to Existing O365

Hi dalbright 

As far as I'm aware, there isn't a easy native way to join to different tenants together, though as more businesses sign up to EOL and this becomes more common, I'd hope that MS introduces a mechanism to do this (as well as splitting up a tenant).

The only thing I have in my notes is a document from on how to do this manually

https://docs.microsoft.com/en-us/Exchange/mailbox-migration/migrate-mailboxes-across-tenants?redirectSourcePath=%252fen-us%252farticle%252fHow-to-migrate-mailboxes-from-one-Office-365-tenant-to-another-65af7d77-3e79-44d4-9173-04fd991358b7

I've not done this myself - we were looking into it as a client had the same requirements as you, though in the end they decided to keep the tenant separate.