I purchased a new sub-woofer which requires an RCA jack. I have a speaker wire buried in the wall which has one end at the spot where i want to put the subwoffer and the other end at the receiver. Can I convert this speaker wire to an RCA cable. I saw some ends at Radio shack which sdeem to use one wire and a ground. Can i use one of the wires as "the wire" and the other as the ground?
eh, you *can* yes, but there will be no shielding, and the impedance won't be correct, so the result may be less than stellar. You'd be better off cutting out some drywall and laying in an in-wall RCA line with RCA jacks at either end, or just laying an RCA sub cable under the carpet or flooring, or behind the trim/kick boards around the bottem edge of the walls, and doing it properly instead of trying to rig a so-so work around.
Shielding is probably not that important in a subwoofer cable but you still don't want to use a speaker cable as an interconnect unless it is the only choice. With RF and EMI everywhere in the modern home you might pick up some noise from an unshielded cable but the only way to know that is to try. If you try this you are aware you need to use RCA's on each and and run this from the LFE outputs of your receiver, right? You cannot run speaker level outputs into a line level input and not have problems.
The correct way to solve this problem is to return the subwoofer in exchange for a unit with speaker level inputs. This should gain you a better subwoofer in the deal.
"I saw some ends at Radio shack which sdeem to use one wire and a ground. Can i use one of the wires as "the wire" and the other as the ground?"
I'm a little leary of suggesting patching anything together when the person doing the patching isn't even familiar with the terminology for "the wire". If you've not done this sufficient times to know what you should do, there's a good chance you can screw this up and your receiver might not be too happy with your work - not happy as in stops working.
You can try the suggestion above for running cable around the baseboards or you can try pulling a RCA interconnect through the wall. Use your speaker cable as a pull wire and make very secure but very tidy splices and pull very gently. If you get stuck and pull too hard, you could end up with no cable at one end of your wall.