I suggest you just replace the whole cable. Its like three dollars worth of wire if its just by your panel. You can't by code run a separate wire to the box, and hell if you're going to do that you just well run the corect cable. Old dryer recpets run "ungrounded" two hots and a neutral. The basically ran 10/2 romex and used the bare wire as a neutral. If you only have one electrical panel in your house and that panel has a main breaker in it, the place where the neutral and grounds terminate is the same place. so really it doesnt matter that much, whats is more important is what type of load you are looking to connect to that circuit.