But in terms of EXIF, problem solved easily, just scan your slide or neg with a DSLR and macro lens...;-)
I suspect the contest entry form that is submitted with your photos will ask you to certify that you have complied with the contest rules. To me, it would be a matter of personal integrity not to lie by trying to pass off images that weren't taken with digital cameras.
It's Nikon's contest so Nikon's rules apply. If you don't like the rules write Nikon and tell them.