Not being able to put a stock on a hand gun seems to me to be one of the STUPIDEST rules around.
Oh, I won't disagree in the slightest, BUT, you have to understand the underlying law the regulations are used to enforce.
And that law is, essentially, that there is a legal minimum barrel length for a rifle, and shotgun. Also an overall minimum length. Anything less than the legal minimum, and it's a "sawed off" shotgun or rifle, and subject to NFA regulation
SO, it boils down to legal definitions. Handguns do not have shoulder stocks. Rifles, do. By attaching a shoulder stock, you have created a rifle, and since it still has the pistol length barrel, it is an NFA item. (Adding a stock and a 16" or longer barrel makes it rifle, and since it has a legal barrel length AND overall length, it is regulated under the usual laws, not the NFA.)
Note also that historical curio & relic pistols and their shoulder stocks are in a different category and have different rules.
ALSO, note that over the years from 1934 when the NFA went into effect, the Treasury Dept. has changed their interpretation of the rules more than once.
The best thing to do is contact the ATF (BATFE) and get an answer from them.
You must get approval, and have it registered, and the tax paid, BEFORE you create the SBR, otherwise, you are breaking the law.
be aware that you can legally buy and own the stock, if you don't have a gun it fits, but if you DO own a gun and a stock that fits it, even though you never put the stock ON the gun, you might be liable for prosecution.
The operative principle is "constructive possession". Meaning, if you have all the parts to make the illegal item (unregistered SBR) you could be guilty of violating the law, even though you did not actually assemble it. That is something a jury will decide, and, personally, I think that is a situation to be avoided.