There is only one file type that uses the .NKIT extension:
Nintendo Kit Game ROM File
An NKIT file contains a complete copy of data extracted from a Nintendo Gamecube or Wii game saved in the Nintendo Toolkit (NKit) format. It is a compressed .ISO ROM file that reduces the excess and unnecessary data in a ROM file. NKIT files can be played on a computer with various emulation programs, such as Dolphin, Swiss, and Nintendont.
The NKIT format was developed to reduce the size of ISO files that store an exact, uncompressed 1:1 copy of a Wii or Gamecube game, which can grow to be very large in size. You can convert ISO files to NKIT files with NKit. You can also convert NKIT files back to ISO files with NKit.
NOTE: Downloading NKIT files containing Wii or Gamecube games is illegal. You should only dump Wii or Gamecube games from discs that you own.
Updated: September 25, 2020