Prob wouldn't be that hard. I used one of those high low converter boxes on my car when I didn't have any RCA's. Just took it from the rear speakers, which I wasn't using. It sounded good too, enough bass to keep me happy.
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.