I did that to drive my nx-studios and my svs subs. If you have some basic soldering skills, it's quite simple. From the speaker wire into the converter box, have a ¼W 10k resistor on the + side from speaker wire to rca plug, and a ¼W 1K resistor across the + and ground of the rca plug. Then just a regular rca cable from adapter box to sub rca input.
If you need to attenuate the sub level up or down, just lower or raise the value of the 10K resistor. I ended up going with a 22K to allow some level of adjustment for level matching. I think I have a photo somewhere that I'll add to this post if I find it.