First make sure that the actor isn't already in use by the base game , if it is then duplicate it in the Object Window so that it is separate, otherwise you will add the item to every one of that actor in the world. Now double click on it in the actor tab (or double click it in the render window and click Edit Base), then go to the inventory tab and drag the item you need to add from the Object Window to the Inventory Box and Tadaa, Done.
I know I could do that, that was the initial plan. I was just stumped over how they managed to do it on a generic leveled list without affecting the base.
Now I've run into another frustrating problem. I'm trying to make a Draugr caster boss that uses fire spells. But when I add the fire spells (flame, firebolt, flame cloak) to the spell list, I can clearly see he equips the fire spells in his hands but refuses to use them. I tried all sorts of things to make it work but to no avail. Did some "troubleshooting" by using fire warlock as a template. It seems the actor stops using fire spells as soon as I change the race to draugr. I checked in the DraugrRace settings, and I can't find anything that seems to be related to this problem.