Can dried out joint compound be rehydrated?
Drywall mud is so darn cheap, why would you do this?

That said, I actually tried this myself one time because I was temporarily without a car to get to the store, and I was impatient.  I managed to get it to a somewhat workable consistency, but no matter what I did, I couldn't get the lumps out of it.  I think if I'd let it sit and re-hydrate over a longer period, perhaps I could have worked out the lumps.  But by that time, I'd have the car, and I just threw the lot of it away.