So I had a problem with shadows appearing where they shouldnt. When I attach the vanilla model to the custom one, either the smoothing breaks or the shadows appear instantly. Attaching the models in ZModeler doesn't mess up the quality, but sadly I can't get a new license right now.
How it looks: https://imgur.com/xXt3dyP
How It's supposed to look like: https://imgur.com/JOVfRHC