I think this is impossible.
In principle, you should be able to write an OpenNI module for your stereo camera that provides image and depth generators, and hook that up to NITE through OpenNI. There's a thread about that on openni-dev here you might want to follow up on.
In practice, I think (it's possible I'm wrong on this) that NITE links directly against the Sensor module for PrimeSense's chip instead of going through the OpenNI middleware. In that case, you can only use NITE with devices using PrimeSense's technology (Kinect, Asus Xtion Pro, etc.).