You can connect a D-Tap power source to your Nikon cameras using the Kondor Blue D-Tap to Nikon EN-EL15 Dummy Battery Cable.
It features a D-Tap connector on one end and a dummy battery on the other end that can be installed into your camera.
It is a coiled cable that can be extended to a length of 36 inches from 16 inches, supporting 12 to 17 VDC input power.