Add new domain to EOP but want this domain to be 0365 plan

I have some domains of my on-premise exchange server secured by EOP.

AD-sync works perfectly

Now I want do add new users of a subsiduary with a new UPN-domain to an O365 plan but keep the user administration on my on-premise domain.

Can I do this (purchase and add licenses to this synced users) and just reconfigure the connectors?

Or do I need to setup an own O365 tenant?