You cannot add a stock without registration and a $200 tax, because then it would be a SBR.
This is a pistol as configured, so you can add a pistol brace without running afoul of the law.
Stay safe & have a great day!

Hi Karmail and welcome. you can add a stock; however, the classification would go from a pistol to a short, barreled rifle requiring NFA registration and a $200 Tax Stamp. You could add a Brace and keep it as a pistol. cheers, stay safe and enjoy!