Hi Davewalter,
Here's my answer. As you mentioned you want to add or delete items from your existing workorder, for that you can go to Field Service Center ->WorkOrder List-> select the existing work order. It will give you Add/Edit Work order, from that screen you can add or delete the items you wants, in that way you can make changes to your work order. Let me know if it helps.
Regards,
-Inder