IMAP Mailbox Mover?

I work in mostly Outlook/Exchange environments. Due to upgrades and new hardware, I have had to move a number of user mailboxes around outside the timeframe of the bulk-move migration effort. Knowing what I do about IMAP, it seems like their could or should be a tool for using IMAP to completely copy a mailbox between two servers. It even seems like it would be an excellent solution for moving a mailbox from another OS/mail system (e.g. Linux/Sendmail).

I did a search and found that someone had written a PHP script to do this, but could not find anything else. Is there such a tool out there for Windows? Or is there a way to use (or misuse) one of the email clients to do this?