You can add a brace but not a stock. The stock makes it a illegal NFA Short barrel rifle unless you do the correct paperwork and pay for the NFA tax stamp

Hi Eric, you can add pistol brace. If you add a stock it will be a SBR, short barrelled rifle, and require registration and $200 tax stamp.
Pistol brace available here >>> https://shootersgate.com/product/sbt5-22-pistol-stabilizing-braces-hk-bt/
Stay safe & have a great day!

Hi Eric and welcome. A Brace would not require any $200 Tax Stamp nor NFA registration. https://heartoftexasarmory.com/sb-tactical-sbt5-22-pistol-stabilizing-brace-hk-mp5-22lr/ ---- Cheers, stay safe and enjoy!