Set the view finder image to monochrome/black and white. You can then compose in monochrome/black and white. Shoot in Raw for a color image and convert to black and white.
Set Picture Control to monochrome. Shoot in raw, and you will get color in the NEF file when you download to computer (which you then have to convert to B&W). Preview of shots, on camera, will show B&W (as the previews are of jpeg). If you shoot raw + jpeg, the jpeg will be in B&W.