Sure, you can get a $500 adapter to mount Nikon glass on a Sony body. There may be advantages (you already own the lenses, it throws a shitload of light onto a small area of the sensor) but there are also disadvantages (running at less than half of the resolution the camera body offers, fully manual operation). I personally wouldn't choose this route for a fancy body like the A9, but I think it's a reasonable choice when using a cheaper MFT body.