If you want to just test migrations w/ the
exchange migration wizard & generally see what the users think, I would just create a 2nd/sub domain and fork/forward all the exchange mail to the zimbra server so they have some mail to play with.
If you don't plan on doing the migration changeover in one day/all at once, you would use a
Split Domain - Zimbra :: Wiki
ie: Some users are still on exchange and some would be on the new zimbra box.