If you can find a link to that walmart camera, that would help. The cheapest currently made and regularly available Nikon DSLR that I know of is the D100. It is a lot more than that price, so clearly this is something else.
The chip that the SD9 uses is revolutionary (Foveon) but I'm starting to think that the market passed it by. Pity, as it seemed to have potential. The real problem is that it only accepts Sigma lenses (maybe there is an adapter?) Because of that limitation I think that:
a) It is doomed. Take only sigma lenses was not a good design choice.
b) I'm not sure if you'll find an adapter for that camera which would fit on to that night vision system. I don't believe that mount is popular (anyone? I'm ignorant here, but I've also never seen anything which takes it.)
This is why I asked what camera mounts the system takes. Probably Canon and Nikon. The Canon D30 is probably in that price range used. If you don't know, could you post a link to the product that would mount a SLR to a Litton NAV-3?