unix auth migrate to qmail vpopmail

I have approx 250 email users on one server which are all unix accounts 
(with uw-imap)

I have a new server that I have just setup qmail/vpopmail/courier.

Does anyone know if it is possible to keep the passwords for users as I 
migrate them to the new server with vpopmail accounts?  This part 
doesn't concern me as much as the next part.

Migrating mailboxes to vpopmail accounts. I haven't done this before, 
what sort of things should I look into for handling this migration?

Any tips and/or tricks would greatly be appreciated.

-Robby