NOT Legal. No, no, no. NO stocks on pistols. That is, without approval from the ATF and payment of your $200 NFA tax stamp. Adding a shoulder stock to a pistol makes it a Short Barrel Rifle, which is a Class III (NFA-regulated) firearm. Doing this would be a Federal felony, punishable by 10 years in Federal Prison.
Additionally, California doesn't allow SBR's no matter what, so you cannot go through the legal steps to get this in your state anyway. Federally they are okay, but the States can still ban them. Unfortunately, even in WA here SBR's are not allowed. I do have a suppressor now that I have moved here, and some other NFA items are okay, but no SBR's here either
Now... all of that said, you CAN
have a folding (regardless of over, under, or to the side) stock on an Uzi or Skorpion IF
if has at least a 16" barrel and, with the stock folded, still meets minimum overall length requirements of a rifle. In this case, yes, it would be a rifle. You can purchase lots of Uzi variants that are legally rifles. This is the only way to have a shoulder stock and avoid NFA regulations.