In the kingdom data, where you specify names, roles, etc… You also point to a model file.
The ascendancy is at /services/server/population/data/ascendancy_population.json
For rayya and north alliance they are at their own mods, at /data/(kingdom)_population.json
That all said, there is a mod group working on dwarfs right now if you are interested in joining: ACE - Authorized Community Expansion Project