If all you have is 12', why not run an all new grounded circuit to what you want, and leave this one alone?
That's actually what I was going to do when I found the junction box. I'd pulled the drop ceiling out below the receptacle and found the junction box and thought maybe I could just leverage the existing romex. I originally was going to upgrade the entire circuit but a decent amount of the wiring is behind finished walls/ceilings. I will eventually fix it but grounding just the one receptacle is a higher priority (and an easier fix).