CC Help How to assign a mesh to an existing bone Blender

[Taddstrange]

I've been struggling with this mod for a while. I've been attempting to make a new nude bottom for wicked whims by simply attaching an appendage to the default sims 4 nude bottom. After talking with community members I realized that I need to attach the new mesh to an existing bone and/or vertex group. I'm new to blender and I have no idea how those commands work or where to find them. After searching the internet for hours trying to find a solution to my problem I've kinda given up on getting a solution from google. None of the solutions I've found work with the version of blender I've been using which I believe is 2.79, everything's for 2.80 and won't register at all. I just need to know how to assign this new appendage to an existing bone on the default female nude bottom so that it will hold the model in the correct orientation in game. I'll leave screenshots of the problem and the file below. I'm putting off texturing and UV mapping until after I've figured out how to get the model in the correct orientation. I just need it to stay in the orientation shown below and in the problem file I attached.

 

The first image shows what happens to the file when I simply join it to the default female nude bottom. It basically maps it to every single bone group in the female bottom and I can't remove any of those groups from the model's mapping as its now part of the default nude bottom. In addition, for some reason upon dragging my cursor across appendage it triggers sliders for the bottom of the sim's foot. No idea why.

 

The second image is where the appendage needs to sit without moving. Again I don't know how to add the vertex groups of one model to a completely separate mesh. I know the parts need to stay separate, can't be joined, and yet the new mesh needs to be assigned to a bone group on the default model from the sims 4 studio.

 

The third image shows the empty vertex groups field I need to fill using some commands that I can't find.

 

If I'm an idiot and just need to use a newer version of blender, or if you know how to fix the issue please let me know. Thank you for the help.

Problem File.blend