From what I can understand that's just not possible.
In order to work and create a moddable eviroment, 7thHeaven patches the Steam version of FFVII (which has achievements coded inside) in order to roll it back to the original CD version (that's what the Game Converter is for) that came out quite a lot of years ago, when achievements weren't even a thing. So the achievements can't be put back as with the version of the game you have after applying the Game Converter they are just not in the game anymore, they do not exist.
On the other hand, FFVIII uses an entirely different modding process of directly changing the textures via hard substitution of the files and by applying costum hashmaps for the game to be able to understand the usage of the new textures via an external tool (tonberry), which leaves the base game code (and thus achievements) basically untouched.