Installing internal chokes with welded on tube?

[Grendel 0]

My S-12 is one of the early batch that came in with a barrel under the legal length and had to have the cylinder-bore external choke welded on to make it legal. Is it possible to have this barrel threaded for, say, Remington choke tubes so that the end of the tube will sit flush with the end of the muzzle like on a "normal" shotgun? Or is my only option to forget internal chokes entirely and have something like a PolyChoke welded on in place of the external tube (to make legal length)?

 

And if it is possible to thread a barrel like this for internal chokes, who would you recommend to do the work?

 

(here's how it stands now - I'm ordering a synthetic buttstock to replace the wood one, and possibly looking at sight mods down the road)