Just tried modding it via UABE. I chose some map and decided to change one texture to see if it would work at all. I saved modified map as a new map and tried to open it in game. But this map doesn't work at all although I just changed one texture. Quite strange thing.
I've always had mixed results with texture modding in Unity assets to begin with, let alone compressed assets.
I don't think you can add more items, you would have to modify an existing item.
IE, change a waifu's skin tone or hair color would likely be possible.
Adding an entirely new model isn't, because the game as a whole wouldn't register it.
I could imagine that a plugin for Bepinex could be designed to sideload a new model, but it would be pretty damn complex.