How do I install an electrical outlet on a live circuit?
There is a reason they put a blank over that box. They do not normally put outlets in a closet, as cords plugged into them tend to get buried in junk if close to the floor or a shelf, creating a potential overheating and fire hazard.  Under no circumstances should you attempt to wire an outlet to these wires hot. It is never a good idea, even if you knew what you were doing. SO DON'T!!!!