Do you want your technicians to be able to add an item from scratch? Would you have them deciding what item type to add and have them selecting from your chart of accounts? This could create accounting problems. And once an item type has been selected, it cannot be change.
If no, we have a recommendation.
Add one or more Non-Inventory type items to serve as “catch all” items. You might number them as “Non-Stock” or “Not-Listed”. The user selects this item, sets the retail price, and adds a relevant description for that item. The customer does not know the difference. Create a User Filter in TOM, so that people can easily locate these items. The office adds the item (if needed) to the Invoice Item List and replaces the Not-Listed” item with the correct item. You can replace these items on an invoice that has been paid. You just have to make sure the retail price remains the same after your replacement.