We have a client use case where they initiate their sales transactions using Sales Orders. Sales Orders are created with both subscription and non-subscription items in the same order. Invoices are generated from the subscription and the Sales Order. Mid-contract, their customer may want to purchase additional items to be part of their subscriptions. I tried creating an add-on to the subscription in draft/pending activation status but I cannot select the same subscription on the new Sales Order.

Please advise on the best way to add additional subscription items mid-contract using Sales Orders.