i was looking into the same thing.
not only do the threads not always fit but also a paintball co2 container had the pin that has to be pushed in which the regulator can't do. I found this which should do the trick ...
1/8 NPT ON/OFF Ball Valve [FITT016] - $16.00 : Palmers Pursuit Shop!, Where Custom Still Means Something
(you can also look for a "remote line fitting")
only thing i'm not sure about is if the male side is long enough to fit on the regulator and have a tight seal. If not then u will also have get a brass pipe adapter and a small piece of tubing.