To add custom items, I normally never use that tab. Not sure what it does.
Anyways, to add a custom item to the NPC's pack inventory, simply drag an existing item from the column on the left into the Pack Inventory box and modify the template resref to point to your custom item. To make the NPC have an equipped custom item, double-click on the relevant equipment slot and type the template resref of your custom item into the text box that appears.
And in case you're wondering, the Pack Inventory contains items that will be added to the main character's inventory once the NPC joins your party and the equipment slots contain items that the NPC will actually use in combat. Both are accessible to the main character if the NPC is killed.
- Star Admiral