I got frustrated with this whole issue earlier too. Like several ppl above suggested, I went the IMAP route and love it. I choose BincIMAP since it is small and simple; It uses the maildir format as its storage so in your case you do not have to convert anything. Use fetchmail/cron to poll your various mailboxes and d/l the mail every minute or so. You can now swap back and forth with ANY email client that supports IMAP as much as you like (KMail, TBird, OE, etc) See this link for a little more detail:
http://forums.whirlpool.net.au/forum-replies.cfm?t=316099