You can submit manipulated images, but the original source had to be a digital camera. This makes a strong case for the suggestion of others, that Nikon rigs their competition towards their own digital gear and that this whole thing is nothing but a marketing ploy, and definitely not a serious contest.Images that have been retouched using software or by other means will be accepted.
The most devious thing you can do to these things is: ignore them. These contests are a marketing campaign and will fail, if fewer good images are submitted. Throwing a tantrum about this will only help them, since common people will only hear "film bad film bad Nikon digital Nikon digital Nikon digital". What they should hear instead is "Nikon contest digital yawn"!