You can look for a twist lock freak barrel kit. There are a few around. It would probably be easier to fine a cocker set, though. You can use a cocker to twist lock adapter. There are some nice ones made by Doc's Machine Shop. The easiest method is the ULE body.
You can check to see if your rail will accept a back bottle by removing the valve and checking to see if the bolt cavities are machined into the back of the rail. A back bottle is ok for CO2, but a bit cumbersome for compressed air. If you are planing to use compressed air, you would be better served by installing a grip frame bottom mounted ASA. If you are planing to upgrade to an RT valve at some point, you will need compressed air anyway, because they can't use CO2.
Except for the Automag in front, its usually the man behind the equipment that counts.