In theory there is the capacity to add 12 new Units, due to room for a possible Dark Omen Addon that sadly never happened.(See dwarf sprite post for details)
However, its much easier to overwrite existing files and then to flip between original and Mod armies (Future Mod Installer or manaully copy and paste). Any sprite can be used as its own 16 colour pallete will be modded with any new sprite, it just depends on selecting a suitable sprite that has same stances or movements but i can easily adapt the new Giant's Frames to work on the Troll or any other sprite.
To learn more check out my other Sprite posts and
http://wiki.dark-omen.org/do/Updated_Sprite_Format
We can add as many as we like by overwriting existing ones, adding all races/units and switch via the Mod installer.