Good afternoon, @cmcquillan.
Thanks for stopping back by the Community with another question about sales orders.
I can see how beneficial this could be for your business. However, adding another sales order to an existing invoice isn't available in QuickBooks Desktop.
I'll send over your feedback to our Product Developers so they can consider this in the near future.
I hope this helps answer your question. Feel free to come back if you have any other questions. Take care!