Accepted domain onnprem Exchange and EXO

Hello,

our customer has primary domain (primary.com) with Exchange 2016 Hybrid. 

Later they bought a new domain (secondary.com), added to O365 and configured some mailboxes wih @secondary.com domain. These are cloud-only mailboxes. This domain is not added to onprem Exchange accepted domain.

Is there any risk if I add this secondary.com domain to onprem Exchange accepted domain? (for example, existing cloud-only mailboxes will be deleted or something problematic thing?)

Thank you,

Zoltan