Yes, that is possible. https://www.warzone.com/wiki/Mod_API_Reference:TerritoryModification
SetStructuresOpt Table<StructureType (enum),integer>: Sets the number of structures, such as cities, on this territory. Nil to simply make no changes.
This sounds like a good mod. What kind of exact behavior did you have in mind?
(Host sets some territories to start with cities? Different ways to get a city during a game? Cities can be removed by bomb card? Cities can grow?)
I think there is a lot of potential here, but let me know what you think would be cool and I might make such a mod.