You can *not* "edit" an NCS file. That's a compiled script. What has to be done is.. if you have the proper source file(s) -that being the NSS file of the same name as the NCS you want, AND all the files listed (if any) as include files for that file (also NSS files)
Anyway.. if you have all those, you edit them to suit your needs, then you need to use NWN Toolset with the Kotor! mod (By Li'L Jawa) to compile new versions of those scripts (the NCS files).
If you haven't got NSS versions.. you have to know a fair amount about coding in general (a good understanding of C based language would be rather helpful for these), and it would be advisable to look at a lot of the NSS files Bioware actually provided us to see what they did, where, how, etc..
Personally, I consider trying to build a script to replace one that actually gets used in the game already from scratch with no source to go from fairly advanced, since without some guidelines, you'll be struggling along and have to figure a lot of things out by trial and error and comparison back to source you DO have for other scripts.
I don't know what Li'l Jawa's stance is about releasing the NSS source for his mods, but it probably wouldn't do any harm to ask. The easiest way to do what you outlined above would be to obtain that (with the tools and other components I've already mentioned) and after changing a few item references, recompile and replace the NCS with the new one. But that'd depend on his answer of course, which I cannot give for him.