Most of these don't have the files set-up to work in SP. They need one of two things to work. 1. NPC file or 2. SP support at the character creation screen.
Here's a easy tool to make npc's it's really quite easy.
This is a little more complicated, but not impossible to create skin files for SP character creation screen.
There's a weight transfer in blender that gives you a quick rough start but you'll still have to fine-tune parts. Which can be quick or a very long and difficult process depending on the model.