Welcome to the forum!
Like John said, you might get hum because the speaker wire isn't shielded like a regular subwoofer cable is. For higher-powered signals that are used for speakers, it typically isn't an issue - but it might be for a line-level signal that you'll be using for your sub. Some people here have done what you are asking about doing without any hum, while others have gotten hum. It never hurts to try, though.
You can make something yourself or buy something premade. If it were me, I'd buy an inexpensive cable with RCA connectors on both ends, cut it in half, strip the cut ends so that the two internal wires were exposed, and then use those two halfs as the adapter cables (end one would have an RCA connector on one end and two wires on the other end to connect to the wall plate).
In terms of premade cables, there are different options available such as
this cable from Radio Shack or
this cable from Crutchfield.