We didn't have our students on Zimbra, but did move our students from a different system to Google Apps and offered migration. We didn't migrate all users but setup a web app where students could request migration themselves.
The way we did this though is we required students to re-enter their passwords. This added their password (in clear text - but only temporarily) to a text file. This allowed us to use imapsync for them with their actual password on our local system and move their mail to Google that way.
State University of New York at New Paltz