I don't think you can do this kind of mesh work in NifSkope. You are going to need something like Blender or 3ds Max. I'll describe how to do it in Blender since that is what I use.
The first step is to load the slave collar mesh into Blender and unparent it from its armature (skeleton). Delete the armature, and save the slave collar mesh either as a nif or as something else that you can easily import (like obj).
Next, load the super mutant mesh into Blender. Then import your saved slave collar mesh (nif or obj). Edit the mesh to move/rotate/resize as necessary to get it into position on the super mutant's neck. Parent the slave collar to the super mutant skeleton, and weight paint it so that it moves properly with the super mutant mesh. Now make sure to select all, and export your new nif.
Blender doesn't export creature/clothing/armor meshes properly, so you'll need to go into NifSkope and fix the shader flags for each mesh piece.
Now just add your new super mutant creature as you would any other new creature.
The mesh rigging is basically the same as what you do for any armor or clothing mesh, just using a different skeleton, so any armor or clothing tutorial should be able to show you all of the details about that.