I don't think that you can do this. I know that sometimes copper is used for the final connection to the equipment, but when it is used, it is connected with flare fittings, not sweated joints. I believe that the odor used in the natural gas to make it easier to detect small leaks is corrosive to copper.
A long time ago when I was in high school, I worked part time at the local hardware store. On a very cold winter day a customer came in and bought a 50' length of garden hose and some adapter fitting from hose thread to pipe thread. As he was going out the door, I just had to ask why garden hose in the winter? His reply, "My hose is cold and I'm going to hook up a gas space heater on the other side of the basement".