Yes, this is common during migration when you have both systems up and running. The most common way is to create a sub domain (notes.contoso.com) so all users with a notes mailbox will have a mail address with that domain (user@notes.contoso.com).
In Exchange you setup a connector for that subdomain so everything that is sent to the domain notes.contoso.com is forwarded to notes servers. And do a similar configuration in Notes.
