I have an AR Pistol (Carbon-15). I suppose I could marry it up to a lower from a carbine/rifle. It would be illegal without the paperwork.
But, regardless of legality, why would I want to put a buttstock on my AR pistol? If I want an AR SBR, I'm going to want something that has closer to a 12" barrel, not a pistol barrel. The fun of having an AR pistol is the fact that it is a pistol! I shoot the heck out of it, works great with brass ammo; chokes on wolf if I don't keep it real clean and lubricated.