Currently it's not really possible to properly map a heightfield to a planet. Even if it were possible there would be very significant distortion at the poles. The best approach would be to export in a 16 bit image map format, remap the image to a spherical projection, then use an Image Map shader with spherical projection to load the image. There should be a 16 bit image format available to preserve accuracy, but I must admit I'm not sure what the best option there would be.
- Oshyan