I'm sure you'll find several capable gunsmiths (makers) capable of making the conversion, but a 600 in a single shot is going to be light on weight and a brutal ride for the unconditioned (and even for most of the conditioned as well.) Worse yet, you 'll probably end up with a muzzle break on it, which will make the experience argueably even worse.

There are several smaller "nitro expresses" availible (with easier-to-find and less expensive components) that would no doubt result in more pleasant ownership and shooting of such a gun.