my .02, but i would think it's legal.
It's illegal to take a rifle, chop down the barrel, remove the stock to turn it into a pistol. But it is not illegal to take a pistol, and add a bunch of stuff onto it (with the exception of a supressor).
This is the reason why when you register a firearm with the option to be a pistol or rifle, you should choose pistol (like the thompson center encore). Because if you register as a rifle, and do the pistol conversion, technically you can run afoul of local PD and the ATF.