heh making entirely new things needs a lot of things done,it needs a button built into the menu (means u gotta edit the interface) a entirely new mesh built from scratch or edited off another unit and then of course skinning it then programming in stats and animations unless again u can copy a modify other units things but then its not really a new unit now is it? suppose you could make spearman and just copy the swordsman animations and edit their meshs a lil. may be hard work but it can be done in time
I found I file that looks like it's got to do with the army, but I don't know, take a peek for yourself at
C:\Program Files\Lionhead Studios\Black & White 2\Data\Balance
Scroll down till you see a file called "GameBalenceArmy", it looks like it's messing with army unit's stats, but it's got a bunch of numbers that looks way to complicated for me to figure out, so if you have some free time, feel free to poke around with it. (remember, all good moders make back-ups)