pine uses the mbox format, which is the same format that Mail.app uses. So, install pine, and then replace its mailbox files with softlinks to the ones Mail.app uses.
e.g. ln -s .pine/inbox.mbox ~/Library/Mail/POP-me@someserver/INBOX.mbox/mbox
Note that it's been a while since I had this set up, so I don't recall where pine puts its mailboxes. The way I did it was to set everything up, and then see what files got created.
If I recall correctly, after pine had read and written the mbox file, Mail.app forgot which messages were read, so it thought I had 1,202 unread messages or whatever.
Now, this was under 10.0 and 10.1; in 10.2 Mail.app has changed the layout of its mail folders somewhat, with a few extra index files and the like, so maybe it won't have that effect anymore