There is really no way how you can convert a JPEG picture (.jpg, .jpeg, .jpe 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 JPEG, 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 jpeg to nef conversion is really not possible.