How do I add an additional part to a custom body?

[persephonecharming]

I have a unique player race mod (not one of the ones available on Nexus, I'm afraid) that works by means of custom armor addon skins (as, I believe, is typical). What I want to do is attach the Khantu prosthetic from Brokefoot's UNP Mashup to the hand model so that it's part of my character's body, rather than a piece of armor (both for roleplaying reasons and to free up the armor slot).

But for the life of me, I can't figure out how to do it.

 

Things I have tried:

• Simply copying the NiTriShape from Brokefoot's model over the femalehands_0/1 and 1stpersonfemalehands_0/1 models in NifSkope.
  
• Importing both femalehands(etc) and the prosthetic model into Bodyslide, reweighting it, and exporting it.
  
• Using the default femalehands(etc) and prosthetic meshes, and attaching the prosthetic to the custom race skin as its own unique ArmorAddon (slot 59, which the wiki lists as "59 - arm primary or outergarment or right arm", if that makes any difference).

I've double-checked in the CK to make sure that all the ArmorAddons and models are correctly pointing to each other, but no matter what I do, the model won't show up in-game. Does anyone know why, and if there's a way to do this?