NCF – Extended list of Neutral creatures.

As far as I know, it was found in the .exe file even for version 3.1, but there's no point in increasing the number... imagine, the AI randomly chooses, say, id_222, but there's no functionality for that element... and this skill will take up one slot for the hero as a useless placeholder!

The link with instructions on how to add a skill indicates that you should only change the values for Quantomas's .exe file, i.e., version 3.0. The scripting mechanics will work normally for the AI as well, so if it's really possible to add new IDs (255 will be fine), I would gladly add something similar to version 3.1.