You cannot (should not) produce a crude from a jpeg. Theoretically, this would be possible because the compressed NEF is based on a TIFF container and a "large" JPEG / JFIF IIRC variant.
And all is not lost because, after having organized this kind of competitions, I can say that you can always participate according to the type of competition and why they want raw files …
The requirement may be there because the contest is for unmodified images – this is common with photojournalism contests where only general or minor changes are allowed.
More likely, the requirement is there to discourage cheating, which has plagued the amateur / competitive world. Assuming everyone takes raw photos (and some people don't – and no, I never understood that), organizers often allow you to provide the original unmodified jpeg on which you based input because actual inputs are usually requested at a specific pixel size.
Some organizations post such requirements under the guise of “ quality '', although this is mainly false because (even now) few devices are able to go beyond 8 bits per channel. From my observation, it is mainly to keep "beginners" at bay. From experience, I have seen many events request a 24/48 bit lossless RGB TIFF for display then on a horrible DLP projector that does not have the tonal response to make a calibration worthy of. # 39; be tempted (if they had attempted one, which is much less likely than you might think).
In any case, even if the rules are listed, they are generally aimed at maintaining fairness and will have a reservation that the organizers have discretionary power over submissions. You must contact the organizers and explain to them that your original has been shot in JPEG as they can always allow you to enter.
And good luck in the competition!