Take out the skill.2da with
Explorer or
Explorer Reborn (newest one)
Look at this link which shows what all the skill colums mean.
skill.2daCopy a row that meets or add a new one. For your new skill.
You will need a custom tlk (talk file) that will hold the name and description of your new skill.
Link of description.
tlkYou can edit 2da with a simple notepad and/or you could use one of the tlk editors which usually can edit 2da also.
There a lot of tlk editors out there look in tools above in the stickies.
You will need to put this into a hakpak
Use nwhak.exe program that comes with the game and is in your nwn utils folder. Drop you files into it and save you hakpak.
Link this hak to your test module. Test the module. This is a general outline, but should get you started. Come back here if you come along any snags along the way.