Unpack game for modding!?

Guys, is there any way to let´s say.. unpack the game files in a way that one can edit some specific file and then repack it so the game just use such file as if it was actually the original? in this way the hackshield would't detect it, if it works that is. no GSL would be needed neither.

Is such thing possible? at least for replacing skin. game skin sucks, don't you think?

If anyone knows of some way, even vaguely, let me know. via private if necessary..