just took one of my old soda stream bottles apart and it apears to have a 3/8th bsp male thread on it underneath the brass adaptor. Unless 18*1.5 metric is a very close compairison to 3/8th bsp. Im sure you could get a 3/8th female fitting from technobots and put it on to use the bottle as a buffer tank.

If you want the bottle its urse, no cost.