I have put swivel studs in several synthetic stocks. Most have thick ribs built in in the propter place for a swivel. Drill them like you would drill wood, put some epoxy on the screw threads, and twist in in place. On the forestock if there isn't enough material, drill a through hole and put a washer and nut on the back side. You may have to shorten the threads to clear the barrel.
Is not hard and works well. Take your time and work carefully.
Good luck!