The procedure is the same in NifScope and NifViewer, but the interface is different. I use NifScope - it's clearly better, once you got some practice.
Open up a unit that has glow (Great Prophet works fine).
Click on the Glow in the viewport to get it selected. Now in the Block View select the Parent Node of it. Should be "NiBillboardNode". Right Click, choose "Copy Branch".
Open the target unit. Find the Node you want the glow to be attached to. "BIP_Head" (The head bone, name might vary) is a sensible choice. Right Click it, choose "Paste Branch".
Now move the Glow into position you want it and Scale it as needed (Right Click -> Transform -> Edit)
You can change color and shape of the glow by editing the Greatprophet_glow.dds (a copy of it should be made in your new units folder).