Glen, look for a DC coupler (weird name, I know) like this...
http://www.ebay.com.au/itm/EN-EL14-A...-/302064843830
I've used similar in my Canons pretty much from the start, as the battery never lasts very long doing long exposures. A tiny voltage regulator from Jaycar drops the voltage from 12v to 8v for dark site use (Canons use ~8v, but check the battery for your Nikon)
The OEM usually do a version at elevated cost btw, so it's up to you which to go for.