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Your best bet for doing this would be if you could have both servers be online at the same time.
What you could do is setup all your users on your Virtualmin server.
Once all of your users are setup on Virtualmin -- look into an email sync'ing tool, that uses IMAP to sync all your email from one server to the other.
I might suggest looking at imapsync -- you can read about using that here:
http://linux.die.net/man/1/imapsync