Good news, this has now been implemented 
We’ve been able to leverage the existing “Additional costs” functionality to allow you to add non-stock items to purchase orders and inventory receipts.
Simply use the “Add cost/discount” button, and choose the line type of “Service or non-stock item”. You can then enter a title for the item, its price and more. I’d recommend choosing the “No allocation” option for the costs as on a service like this (unlike something like a delivery charge), you probably don’t want to spread the cost of the item over any stock items on the document and impact their landed costs.
A PO/IR can be finalised now, as long as it has at least one stock item or one service/non-stock item. Previously there had to be at least one stock item.
We hope this is useful, and thank you for requesting this & helping us to make Breww even better 