How did you put the cat ears on the model?
Actually, in this model I just used the base provided by that Nekomimi Sample link, and tweaked the base model in VRoid Studio. But I noticed that there wouldn't be an easy way to just attach the ears to another model/hair in VRoid Studio (the ears are joined with the main hair).
But I recently learned about Blender's CATS plugin. With it, I think you can easily mix and match parts of a model with another (alternatively, you can use Blender's native features for that). There's a very useful feature that can separate the model's parts!
Some tutorials that show CATS plugin in action: