Adding additional items to an order before making an invoice

Question:
We run a small engine repair shop. When people book a service there are many times when the service is being performed that there are additional items that we need to charge the customer for that were not know before we diagnose the equipment. I do not see that you can add items to the order. If you create an invoice from the order you are not allowed to add items to the invoice either. Does anyone have a way around this?
The only way I have been ablet to accomplish sending a correct invoice is by starting a whole new invoice from scratch. It would be much more efficient to use the order to create the invoice as the customer has already entered all the needed info when booking the appointment and it would significantly reduce steps.

Can surely be modified by code and some workarounds. But without knowing the exact setup → to difficult to give an answer.