Yeah, I’m seeing that now. Joint Ops is possible because of the character_manifest. But that only contains generic human NPC’s (scientists, security, soldiers). From my understanding, it exists to choose randomised character models and loadouts for non-scripted characters from a series of presets defined in the file, as well as to set the permanent appearance for scripted characters.
Anyway, the point is the monsters don’t have anything like that, thus there is no file containing character generation data, thus it’s probably not possible to set multiple models or skins for them. That’s unfortunate, seems the only way to get our hands on a Zombie Soldier is to edit the source code and maps, or hope the team has had a change of heart somewhere between pre-release and steam release.