We've had that question come up in another thread. My immediate reaction was the same as Brian's - it would degrade the image. But the real question is whether the degradation from the teleconverter would be more or less than the loss of pixels by cropping to DX. It would be good for someone to give this a test. Of course, the Nikon teleconverters won't fit, so the test would have to be with the Kenko Pro teleconverter.
My guess is that with 36 megapixel resolution you will see a softer image, so then the question turns to what happens when it is resized to DX proportions.