There is pretty much nothing you can't do to a game you bought on steam. You can copy and modify all the files you download - in fact Valve strongly encourages

There is pretty much nothing you can't do to a game you bought on steam. You can copy and modify all the files you download - in fact Valve strongly encourages doing so! You can even play on multiple machines at once over a LAN by turning off the steam client's ability to phone home. Steam is a more like a walled national park - the walls are there, but they give you a lot more room to run around.