The 72 nations are developer added to the game. And they are all excellent.
The 20 empty slots are fought over by at least twice that many really interesting nations. The cool thing about Dom3 is that it has much more room for uniqueness in its modded nations. We even have one guy turning it into a space game.
As to the modding question, Im more the map commands and server commands guy. But there are definetly commands in the mod docs for modding equipment. Also nations, units, gods, spells, and magic sites.