My mail archives used to be just a zip up of my Eudora folder. I've since moved to IMAP for all of my new mail, and once I get the archive issue solved, will open up all the mbox files from Eudora and toss them into my IMAP server.

Thats one thing thats great about Eudora, it uses the standard mbox format for storage, then just creates a seperate index file of it's own for searching and speed.