after testing things a bit, it's unfortunately a no. the uma system was pretty much gutted from a21. so there's no way to update my UMA style zombies. they will be left to unlive the rest of their days on alpha 20. i do have a fraction of those zombies that don't use UMA which i could recover kinda (there's still a bit of lost details and effects due to alpha 21 changes), but for now will leave it be.
to create custom zombies, the easiest remaining method is the texture swap method that the Snufkin zombies and Robelotto zombies use. they're both great zombie mods that adds lots of variety. of course the most die hard complete way is to create your own 3D models and import using NPCMod.
i'm slowly learning Blender and currently returned to modding Minetest with plans to continue my enZombie mobs there in the low poly blocky style, but who know's... if my 3D modeling skills get good enough, i might attempt to 3D model custom zombies for NPCMod.........