Rigging / Weight painting is an essential process when creating custom models. You have to assign certain vertices to certain groups which resemble the nodes. This can't be done in Nifskope, you have to use blender or Autodesk 3DS max instead. There are different ways of weight painting, using vertex groups, using envelopes and in blender you can also use the weight painting mode. Envelopes are not really good because the results are of varieing quality.
Tutorial: Rigging in Blender