Can i mod/change a game with only the _data folder?

What I'm wondering is if you only have the finished game and the _data folder (take slender for example), can you in anyway mod, edit or otherwise change the game and extract models, music and SFX.

I'm wondering because i want to take a look the mechanics behind Slender's movement to get inspiration for my own game "Project Slender" which is still in extremely early stages of development (like drafting ideas early). I know its possible because i have seen a couple of slender mods, specifically one with Angelina Jolie's face on Slender's, so i was wondering if the guy just got a hold of the source files or if he just modded the game using only the _data folder?