I can't get into the exact details. As I don't have any files to look at right now. But if you open, say the lich creature file and the bb mesh (male or female). Copy the name of the NiNode 0 of the creature. Paste it into where the bb mesh says Scene Root. Then copy the node titled "body," I believe, on the bb mesh. Paste that under the the node titled Bip01 for the lich. The bb mesh should remain pretty much the same I belive, pasting over the lich body.
Either before or after you do this you'll need to delete all the meshes associated with the lich (a golden saint or dremora mesh may work better).
Now, since it's a creature now. You will need to manually place the head/hair/clothes you want the creature to display. Make sure the head/hair are pasted under the "head" node for the skeleton (found under the Bip01 node). The clothes you may need to mess around with, the same renaming of the scene root would need to be done for them, but then just try pasting them under NiNode 0 or Bip01. Mess around with it.
If you don't want to use a bb mesh for whatever reason, you'll need to do what I said with the head/hair for every body piece.
Set the creature up in the CS like a skeleton/dremora/whatever. Remember that you'll need to create the X.nif and X.kf files for the creature as well.
Hope this helps somewhat. Have fun!
-MD