(adsbygoogle = window.adsbygoogle || []).push({}); How do I sync someone else's trophies onto my PSN account?

You can't, you can earn trophies on an account that hasn't been signed up to PSN, but you can't sync trophies from somebody else's account.

The trophys are kept in the ps3 USER. If the user is already associated to a PSN account, it will only sync to that PSN account.
If the user, has trophys earned and no account associated yet, you can login with any account, but once you login it will only sync trophys to that account.
Otherwise people would share trophys all time and would not need saves.