The thing is, you'd have to port the game for that to work. PS1 classics are just the original games, only done for it to work with the PS3 or the PSP, but they're still PS1 games that are being emulated, the actual code of it is still that of a PS1 game using a PS1 architecture.
On the other hand, to add trophies, you have to alter the code itself of the game, because the trophy has to activate somewhere, you gotta put some flags. And how you're gonna tell the PS1 architecture for which the game is done that "after you beat this you get a gold trophy". You'd have to not only add the trophies, but simply redo all the game for it to run not on emulation, but as a true PS3 game. Now, that doesn't mean it wouldn't be worth it for SOME games, but it's not a 5 minute port, more like a full-fledged port of the game, no different than a remake








