I'm sure it's possible since GameMaker 8.1 leaves all it's assets unencrypted, though it might be a fairly hard process to inject sprites into memory unless you know the exact location, and if it is the same location everytime upon starting the game. It may even be different for every game.
GameMaker: Studio for some reason does the same thing with it's assets, everything is left unencrypted except code, which I guess is a good thing. Modding sure is a lot easier in Studio, since everything should almost always use the exact same locations in memory to store sprites.
You can ask Patrick or Klazen for more infomation on this, they seem to know their way around the GMK/GMX File Structure more than I do.