Modifying 3DS models (adding parts) [Pokemon Super Mystery Dungeon, 3DS]

To describe what I'm doing, I'm working on a Pokemon Super Mystery Dungeon mod that adds new starter/partner pokemon (specifically, shinx, maybe others in the future) for personal use. However, after getting the mod to work, I discovered an issue: the shinx model lacks the scarf both the player and partner get in the story.

Due to how important it is, and because I'm too thorough, I want to add the scarf model to the shinx model, so it shows up properly in game. However, I have found no guides that tell you how to add a new set of polygons to an existing model in a 3DS game. So I've been stuck.

I have the models needed (the shinx model + the scarf model), but I can't combine them. I'm wondering if this is possible, and how I'd go about doing it. I've seen mods that add new animations and edit the models, and I figure this is probably possible, but I need help knowing how to achieve it.

So, I'm asking here. I don't know where else to ask, so I figure the romhacking sub is probably the best place to ask romhacking questions. If not, please point me in the direction of the correct subreddit.