Not directly. The order page lets you refund individual items (which marks them refunded and credits the customer) but there is no button to add new meals to a paid order. To add items, place a new order for the customer with Checkout As (Ordering > Checkout As) — a custom charge can also be used to bill an extra amount. For subscription orders, meal selections are changed on the customer's subscription page (the Adjust Delivery Date link on the order takes you there).