There are some OSS solutions to do imap migration and imap mailbox copies For very small user groups you can use a mail client. For larger groups this can be done with such a tool. We don't have a reccomended *best* solution at this time. We welcome feedback of what worked for you. A web search turned up a few links that may be a starting point.
http://freshmeat.net/projects/imapmigration/ http://www.jplanglois.com/products/imapcp/ http://freshmeat.net/projects/migrationtool/
-kevin