In a closet, that is an enroute junction box, which is there for accessibility (perhaps because the building is wired in conduit and they don't want too many bends between accessible points).
Because it's enroute, you have a special problem: you will be breaking splices that have current flowing through them. For instance when you break open the neutral wire-nut to add your neutral wire, the current flowing through them will arc impressively, and when you get them apart, now the supply neutral will be near ground, but the load neutral will be pulled up to hot potential by the current trying to return. All downstream neutrals on that circuit will also be at hot potential, if anyone is bootlegging ground, you just killed them. It's really hard to wire-nut wires that are actively arcing.
It's not intended to have a receptacle there, because receptacles in closets are a bad idea, as the cords plugged into them will get covered in clothing and potentially overheat.
If you really, really want to do this, talk to building management with the following proposal: run surface conduit (wiremold) from the junction, to a correct switch location for a closet light, then up to the ceiling for the closet light. Say you want to use a plug-in light you already own and ask them to provison a switched receptacle up there. Why? Because the ceiling is the only safe place for a receptacle in a closet, and putting a receptacle on a ceiling only makes sense if it's for a plug-in lamp.
You will probably have to spend a bit of money on this, it might help if with permission and inspection you did the fitting of the surface conduit yourself, properly. Though union rules may disallow that.