So I reckon the box is just a few RCA's and some wire connections, Which means you could just solder some RCA's to the speaker wires (for a sub you only need one though, mono. Then get a splitter if you want) And plug it in.
I don't know if the speaker you took the signal from will still sound strong enough afterwards though. And I haven't done this at all. Just Speculating, so no guarantee it will work, but it's not like it's too hard to reverse. I say give it a shot, see what happens.
As an afterthought, Maybe there is something else in the box to boost the signal. but I sold mine so I can't have a look.
Do Not Do This!
That would be the quickest and easiest way to blow up your amp. The converter box will change the signal from a high voltage speaker output to a line level rca signal. Much different from just wiring speakers to a rca plug.
That box jello suggested will work fine, and no problem running them off the rear speaker outs either.