Ah, I did this recently. It's really quite easy. Simply go into NifSkope and open both your unit and the Great Prophet. Click on the "Glow" from the prophet, and copy the whole node. If you just copy the model, it will come out as just a circle. Go to your unit and find where you want to put this glow. Paste it there and move it around until you're happy with it.
Is that clear enough? I may have glossed over some steps.