I don't think imapsync supports it yet, but Cyrus SASL supports the concept of proxy authentication. That's where you authenticate as an admin user, then you pretend to be a normal user. This way you don't have to mess with resetting each user's password.
I used a simular method in a custom Python script when we migrated our users from Sendmail to iPlanet a few years ago.
Another useful tool is Perdition.
http://www.vergenet.net/linux/perdition/
It's an IMAP/POP proxy server. We used perdition to do a gradual migration, and it redirected the users to their correct mail box.
Hope this helps.
Rick