There is no way to push emails back into a POP server via POP.

However, you can get them back there the same way they initially got there, via SMTP (standard emailing). The problem is that most mail tools don't have a way to send an exact copy of an email, only a forwarded copy that has a lot of cruft around the original message. However, the mail tool pine has a command called bounce that will send an exact copy of an email in a mail folder it can see to whatever address you want.

Pine will run on OS X, but I don't know if it can necessarily read the format your mail is in now. What mail tool are you using now?

Assuming you were able to get pine to read those emails, the only problem with bounce would be the potential for the emails to be in a new order and the fact that your regular mail program will probably see the redelivered messages as new messages (and download them again). Some mail headers will change or be added, but they'll mostly be stuff that you'll never really see; the normal ones (From:, To:, Subject:, Date:, etc.) will remain the same.

This is the only even vaguely automated solution I can think of (beyond writing your own tool). If you want to go this way, let me know and I'll give you some more specific instructions.