you can adapt a normal RCA cable - cut off the plugs at one end and use the wires as a left and right. You with the wires, you need to make sure you have the right signal wire and not the grounding wire (the correct one should be in a seperate plastic sleeve).
You will have alot of crap on the amp though - becuase the normal speaker outs arent leveled like the RCA outs.
In short, it can be done above but if you are serious about having an amp and sub you might as well do it properly and get a HU with RCA outs