You can create a purchase order from the FBA Shipment. After creating a FBA Inbound Shipment, click on Create/Sync PO from the Action Menu and follow through.
However, this is only a convenient method of ordering quantity for FBA. Unlike creating an FBA Shipment from a PO, creating a purchase order from an FBA Shipment will not direct the Vendor to ship the items to the Amazon Fulfillment center. The Ship To address does not change to the Amazon Fulfillment center on the FBA Inbound shipment. And if your in-house stock is lower than the quantity you are shipping to FBA, you will not be able to preview the shipment until the purchase order has been received Once you receive the purchase order, you will then be able to complete the shipment.