How do you connect a remote line from a 20oz Co2 tank to a paintball gun?

No. This is because the end head of the remote line does not have a valve. you would have to open the tank valve, fill it, close it, remove the remote, put it in your gun and then turn it back on. This would be more complicated then just removing the tank and filling it. If the remote line had an ending with a valve it would be easy, otherwise removing it would re-empty the tank.