Ryguy is correct. To attach a shoulder stock to a handgun is (with very few exceptions) considered manufacturing a Short Barreled Rifle. This requires paperwork with the ATF, and payment of a $200 tax. Failure to do so is a SERIOUS felony, punished by up to 10 yrs in a Federal prison, and a fine of not more than $10,000.
REALLY bad idea.
What we have here is... failure- to communicate.