Very bad idea,in order to fill a tank you will need a syphon tubed supply and the various bits of plumbing to fill,chill and dump the back-pressure.Fire extiguishers are not designed for this in that the valve assembly is of the wrong type.It is simply not worth the effort or danger-many years ago I saw a valve fail mechanically and the 20oz bottle at the end of the fill remote become a highly agitated helicopter.