Oh, it seems they have removed the feature in Kontakt 6. Sorry, I wrote this based on outdated information. If you have an earlier version, there are threads in the NI forum discussing the feature.I tried but it didn’t work. Not sure if I did it correct but I looked for the folder in kontakt. Found it but no instruments showed.
But honestly, I would just use plogue sforzando. It's free, it works, and you get the custom ui for each instrument as shown in the link posted earlier.