Not sure about 2.4 and beyond, but I recall with 2.2 and (I believe) 2.3 the messages were each stored in individual files. This is how many old mail servers (like Post.Office) store their email, and there are plenty of conversion utlities out there.
If you are adventurous, you could use formail and mailutil to convert mail files from any format into whatever formats your mailutil supports - mbox, mbx, etc. The later is part of the uw-imap suite (c-client, IIRC) and not sure about the former. I do know that most Linux installations have them, and you can compile them on others as necessary (which is what I do for my Solaris boxes.)
