Placing a stock on a pistol with a barrel length less than 16" would be making a short barreled rifle.
The National Firearms Act (NFA) Handbook.
2.1.3 Rifle. A rifle is a firearm designed to be fired from the shoulder and designed to use the energy of an explosive in a fixed cartridge to fire only a single projectile through a rifled barrel for each single pull of the trigger.11 A rifle subject to the NFA has a barrel or barrels of less than 16 inches in length.
3.2.3 Registration by makers. Persons other than FFLs and SOTs desiring to make an NFA firearm are required to first register the firearm by filing Form 1 with ATF and obtaining approval of the form and registration of the firearm. Appendix C contains a copy of the form. ATF will approve a making application on Form 1 if the maker pays the $200 making tax required by the NFA, identifies the firearm as the form requires, includes his/her fingerprints and photographs if the maker is an individual, and if the making and the maker’s possession of the firearm would not place the maker in violation of any Federal, State or local law.34 A law enforcement certification is also required if the maker is an individual.
It's all here. http://www.atf.gov/files/publications/firearms/nfa-handbook/
