adding any stock would make it a NFA device and require a lot of paperwork and a $200 tax stamp as it would then be a SBR (short barrel Rifle) You can add a "Brace" without changing the classification. but physically yes you can add a folding stock but legally no not without a lot of paperwork and some "green"

Only if you plan to apply and pay the $200 Tax Stamp Delma. Once a "Stock" folding or otherwise is added, this will no longer be classified as a pistol but a Short Barreled Rifle or SBR. You can add a "Brace" without changing the classification. So the answer is no unless you plan to apply to te ATF for NFA legalization.