This only works for NPCs, or when you use the playermodel command in SP, I believe. You'd need a good Hex editor to do it without any modeling software, although I'm sure Blender makes it much easier than it used to be. The _humanoid skeleton is referenced in the model.glm file itself. I always use Toshi's Yoda as a reference for this, since it uses it's own. I can't really take the time to explain how to do it right now, so maybe someone else can. Essentially, you open the model.glm file with a hex editor, find the line that defines the _humanoid.gla and change it to the one that you want.