Only software from Native Instruments (KONTAKT, TRAKTOR etc.) can produce .nks archives with protected content. There is really no other way how this type of archives can be created and no 3rd party software supports it.
Convert Native Instruments instruments sample to Native Instruments archive file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert nki to nks on Windows
Audio sampler for Windows
Convert from nki file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.