There is really no way how you can convert a TIFF picture (.tif, .tiff, etc.) to a Nikon Raw image (.nef). It seems that a lot of users are looking for this conversion because they accidentally captured photos as .jpg files instead .nef files and want to additionally apply all kinds of special settings to the pictures. Alas, this is not possible.
When a .nef file is exported as TIFF, all the special metadata captured by the digital camera (light, exposure, time etc.) is discarded and you will get only the photograph.
It is true that Nikon software allows users to save picture projects a .nef, but that is only so you can work upon .jpg file and retain all the previous edits. This will not "magically" create any data, so tiff to nef conversion is really not possible.
This conversion entry was verified by us and conversion was flagged as wrong or not possible.
Usually this happens when the conversion would try to convert different types of data between each other, such as audio to image, or it is not possible because incompatibility between programs, caused by closed file formats, very limited support etc.
If you find entry that you feel is flagged as bad wrongly, please tell us in the discussion below with as much as information as possible so we can fix it.