As far as I am aware, you need to download the mods through the game.
1.  Start the Game.
2.  Click on Mod Browser.  (Internet access required)
3.  Scroll through the list of mods, until you find these two mods: (You need to download both mods, numbered 1 and 2)
	
		
			
		
		
	
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This is the only mod that I am aware of that will allow random species to spawn with anal wombs.  The other mods I have seen, would only mod that specific species.
4.   You might not have any problems if you load an previous save, but you might want to start a New Game.  It isn't guaranteed that you won't have any issues, but it won't hurt to try loading a save.   Any currently known generic NPC will never gain an ass womb, only new characters you meet will have a 50/50 chance.
5.  If you don't want to use that hyper mod, there are specific races that can have an ass womb.  Each species will have (Breed Edition) listed to the right of the species name.
     So:  Skulldog species will NOT have ass wombs for the male/shemale characters.
            Skulldog species (Breed Edition) WILL have ass wombs for the male/shemale characters.
            Loading that mod, will ONLY affect that species, it will not impart that ability on any other species.