Just finished migrating a relatively large Virtualmin/Webmin to Zimbra 7.1.0_GA_3140 successfully.
I've attached my script, extended from
Migrating accounts/users from passwd/shadow file?
It takes in all required files from Virtualmin/Webmin installation (shadow, passwd, ...) and the domain list to migrate giving you scripts to run the migration it self (account creation on zimbra, password modification and imapsync).
Enjoy!