I'm not sure you can do this just with the editors included in game, I think you'd have to look into modding the game. But the modding community for this game is very good. Take a look at the Children of Storm mod by Heavenfall, he has some non-humanoid races such as Centaur, Angels etc.

https://forums.elementalgame.com/443960/page/1/#3357353

You could also try using the faction creator to create a race with very pale skin or skin of the sort of colour you think a Zomibie would have. The Wraiths (Oracle Ceresa/Resoln) already look kind of Undead. Maybe not what you are looking for, but it might be a place to start. Butchermen (who are the closest Elemental comes to Zombies I think) do kind of go down this route.

Hope that helps. All the Best.

Happy to help. If you find any modding tutorials please let me know.

also forgot there is a mod called necromancy extended or something similar to that. its not a race but it gives you the option to make a more or less undead faction. you could try looking into that if you want to play a faction that can produce undead from the cities.