So, after examining the game files, I found a lot of interesting things...
For example, sprites.lod contains the unit animations and the units themselves... All animations in the game are hand-drawn... so adding units won't be a difficult task... EnglishT.lod contains text files... the ones described here, and a bunch of other files... Texts for quests, recipes, descriptions, etc.
EnglishD.lod contains background images, buttons for the background menu, etc.
icons.lod contains armor, icons for game units, weapons, artifacts, etc.
So, with this program, you can create a pretty good mod...