Standard PR/PO can be created only when we are buying the inventory from external vendor. Since you already have the inventory in the Plant (i.e Stock Accounts) there is no need to create the PR/PO.
You may create a stock transfer requisitions from SO if you would like to trigger the movement between Plant and 3PL based on Sales Order. Goods Receipt for STO will then move stock from your plant to 3PL. You can use that stock for SO PGI against delivery.
Regards,
Niranjan