NCF – Extended list of Neutral creatures.

It won't be possible to add a new faction; the game already has a neutral faction, and for it to function properly, its castle lacks a building plan. Even if there were a plan, the buildings would likely not function (like the Mage Guild and Orc buildings – they can be added to the building plan, but the spells won't appear). All special building properties are defined in the executable file.

Well, the Mage Guild can be replaced with other buildings, and the magic schools can be replaced with new abilities, like battle cries and other things.
But if the neutral faction is implemented like any other, then all the neutral creatures will have to be distributed among different races, like the mummies in the Necropolis (in the original game, they are listed there, which is why Markel could recruit them into his army).