You could, but it wouldn't be worth it. There is such thing as a green gas rig, which is basically rigging up propane to the gun and let me tell you, it does not work well. Not only do you have a very small range of pressures to work with, but if you fire too much your pressure will decrease because of cool down. HPA is basically what paintballers reffer to as Nitro, HPA stands for high pressure air, and air is largely nitrogen. CO2 isn't too bad, it's about 4 bucks a tank and depending on your gun the tank can last thousands of rounds.