NCF – Extended list of Neutral creatures.

Thank you for the informative answer. Would it be possible to add a new town and faction in this way?

Adding a faction won't work; the game already has a neutral faction, and for them to function properly, their castle needs a building plan, and even if it existed, the buildings probably wouldn't function (like the Mage Guild and Orcs - you can add them to the building plan, but the spells won't appear). All special building properties are defined in the executable file.