Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
All extensions that I'm aware of are for editing, not the organizational aspects of Photos.

If you want software that can deal effectively with metadata look to something else besides Photos. Photo Mechanic is the best, but there's also Adobe Bridge, XnViewMP, exiftool, and others.
 
What meta data are you trying to change other than file name? I seem to be able to easily change gps, date and time, tags, descriptions, titles without trouble at all. It's only the dang file and that I can't change in photos that drives me nuts.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.