Have you mapped the Port as a Port or Bonded Warehouse in SAP system? If yes, you can first receive the goods in the Port. Here you donot need to pay the Custom duties. Once Goods are removed / transferred from Port to the Manufacturing Plant, you need to pay the duties.
Please note that there would not be any accounting document generated / possible for Inbound deliver against Import Purchase order.
Could you please elaborate your business requirement in detail.