I'm a little late to answer this but a few days ago, this was posted on youtube:
On the comments there is the answer to your question (The video also shows it, but you'll need to slow it down to see since it is a timelapse, not a tutorial)
You'll use skin wrap modifer to copy weights from the body to the mesh... so the body needs to be rigged, like maitreya, legacy, and so on, so you can transfer weights to your mesh that's on top of it. You won't need any add-on like Avastar or Mayastar, they help, but you won't need them.
"Yes, i just use .fbx or .blend files (whichever software you use).. all bodies have skeletons and their "meat" has weights, the creator of the mesh body needed do create that for their body, all you need to do is transfer that info to your mesh.. you can use max, maya or blender with .fbx, .blend or simply .dae files (as collada saves that animation information)"
"You can use mayastar or avastar, i believe it has sliders so you don't have to manually transpose, resize or move bones, but i don't mind it"