You will need to make changes to the battle_modelesdb. This will be much easier to do if you download and use the formated version. Here is an example unit, unmodified.
As you can see, there is a lot of information, but its actually three lots of info, 1 for the basic model (22 dismounted_italian_maa )and 2 more for the armour upgrades (26 dismounted_italian_maa_ug1 and 26 dismounted_italian_maa_ug2). Note at this point its important to understand that the number at the begining of the line states how many characters the following label is, ie. dismounted_italian_maa_ug1 contains 26 characters. The number of spaces also is very important when dealing with this file. If your unsure backup first, in fact backup first anyway it always pays to be cautious.
Here is the 2nd armour upgrade with ownership added for Hungary and Slave (for if they go rebel.) You will need to make changes to all the different armour types if you want it to work correctly.
You can see from the text that the textures are already in existence in the folder the path indicates. However the sprite is not there, so I simply used one that I know for sure exists, from Sicily.
The blue number will also need changing as it represents the number of factions to follow.
If you manage to get all this working you will still need to copy a few TGA files to your chosen factions to give the new owners the unit portrait and info card.
Good luck!
@Edit
I did some poking around and found this formated battle_moddelsdb, it contains all the Vanilla units and a lot of the Kingdoms factions aswell, ignore the title.
http://www.twcenter.net/forums/attac...3&d=1218228276
Chances are, if your using stainless steel the file will be formated already...