It's possible, but you will have to script a bit.
1. Download the Zombies Add-On pack from RTB.
2. Open up the zip.
3. Go to ZombieHorse.cs.
4. Look at the code. Replace all the parts that say RotHorseZombie with RotWompratZombie.
5. Replace Horsedts with Wompratdts.
6. When you have done all these, post again so you can get the rest of the instructions.