In order to create a custom line item, open the appropriate claim, go to the scope of damage page of the estimate, and double click on the area that you wish to add the custom item to.
At the top middle of the Virtual Scope Sheet click on the Custom button. This will bring up a screen where you can define every aspect of the item.
You can click the My DB button at the top if you want to use this item later. Once the item is completely created to you liking, click done in the upper left.