I hope this is not something you all already know:

I decided to give JD's battle model tutorial a try, and made a new model from the greek_city_external_corner_tower.cas. I didn't want to extract all item files to my game folder and delete the pak files.

So instead I opened the descr_items.db file with a hex editor and edited it:
I replaced all textlines with "greek_city_external_corner_tower" into "medie_city_external_corner_tower" (same length, dont know if thats necessary but just to be sure).

I then added a new "ambient" to dbb_walls_roman.txt called medie_city_external_corner_watchtower and set it's item entry to "medie_city_external_corner_watchtower":

Code:
	level
	{
		min_health 161
		battle_stats flammability_none impact_damage_low
		item		medie_city_external_corner_watchtower
		physical_info	info_walls/eastern_city_tower_ec_info.CAS
	}
I ran a custom siege battle with the walls present and the model showed up nicely. I now have a new item file called medie_city_external_corner_watchtower.item

This leads me to believe that all you need to add new building/battle models is to add entries to descr_items.db. I'm sure someone else - smarter then me - can find out the format of that file and how to add entries.
This would make things a lot easier because you nolonger need to repack the pack files to get the new models to show up.