Convert TIFF to NEF

Converting Tagged Image Files directly to Nikon Electronic Format RAW Images (.tiff to .nef conversion) is not possible.

Nikon RAW Images (.nef) are only the first step of a bigger process, basically. You can either keep the file as a raw image, or a NEF file, or change it to a more common format. That is the reason why you cannot simply convert .tiff to .nef.

Furthermore, it seems unnecessary to make such an elaborate conversion, turning .tiff files into raw image representations of photographs taken with a Nikon camera.

In a word, direct tiff to nef conversion is not possible.
So don't expect any so-called tiff to nef converter or tiff to nef free online conversion tool available.