This is probably less of a hint when OS X is set to English, but I found French language on-screen instructions somewhat confusing. Also, there's a caveat or two, so maybe comments will lead to a better procedure.
IMP is a PHP/MySQL Webmail solution and is quite common, at least on university systems. I recently found out you could use it to download mailbox files in standard Unix mbox format. OS X mistakes these files for actual Mail.app mailboxes (giving them mailbox icons), but you can't directly put them in ~/Library/Mailboxes/. I was still able to transfer the folder using the import procedure, but all messages are marked as new. So this might be more of a proof-of-concept thing than the best way to transfer mail. In fact, it might be easier to access the folder through Mail.app and move all messages to a convenient local mailbox ("On My Mac"). In any event, here's the procedure. In IMP 3.2.1, do:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031014173130870