Have you checked out EJB by OReily. I believe they have a chapter that talks about the EJB 2 and 3 beans interoperability. That might help you.
I have one doubt though, when you migrate your entity beans to JPA entities, would you be modifying the existing code base? If yes then why not take annotations?
subject: how to migrate EJB 2.0 to EJb 3.0 using XML descriptors