Nikon makes (2) remote releases, that plug into the front of the camera, below the PC sync socket.
One only has a trigger and a lock button, but it will get you the effect you want, you can just hold the trigger down or depress the trigger and then slide the lock till you are done.
MC-20 I believe is their number, but just look up on the Nikon site for remote releases.
The other and more expensive version has a timer.
Paul Caldwell