How do I add landed costs to a Purchase Order that has been received?

Once a Purchase Order has been received, it can no longer be edited. As a workaround to add landed costs to a purchase order that has been received, Void the Procurement of the Purchase Order (not the Purchase Order itself), add Landed Costs and Receive the procurement again. 

An alternate workflow is to keep the Purchase Order in the Active stage until you know the landed costs, and then Receive the Purchase Order.