Don't know about anymore, but IIRC, PMUA used to allow same day standby for later flights. Of course, I don't know how many people actually tried this - I would never, seeing as if you don't make it on a flight that same day, you were SOL. And that was for same day, OP is looking for a week.

To answer the OPs question, save for a more than minor schedule change, or last minute cancellation of one of the flights, there is no way to extend the stay for a week without paying the change fee that was agreed to as part of the terms of the ticket they bought.