Is it possible to write an application with lite-c that launches an different game and then constantly executes an fx-file over that view? I'm looking for a way to add post process effects to an already existing game. (like someone did a while ago for gta:san andreas)

I don't think this is possible.
You need to apply a dll hack like enbseries did.

Btw enbseries (the gta:sa gfx mod) also works with other games. It even partially works with gamestudio. Just try the different dlls provided by the author and see if it works with your game.

it is possible. you have to hook some function calls, though, and you definately need some good coding skills. have a look here:

http://www.farb-rausch.de/~fg/kkapture/