String is best, but you could try some ordnance survey/gps software that can output a result in nm and use Windoze' ability to cut and paste from one program to another (look for Paste special), in this case excel. Some programs support it, some don't, but the big thing to watch is that the link breaks if the files move afterwards.
The only quote spreadsheet I have around is for contracts, but you're welcome to it if you want to fiddle.
Phil