Why cannot add the new line into an existing Inbound shipment?

Summary:

I cannot add the new line into the inbound shipment. I can create new ASN and update existing lines in the ASN but I can add the new PO lines into the ASN.

I also cannot find the way to use REST API to add the new lines into the inbound shipment.

Content (please ensure you mask any confidential information):

REST API create ASN ⇒ inbound shipment PO to shipment

 /fscmRestApi/resources/11.13.18.05/receivingReceiptRequests

update existing inbound shipment ⇒ Inbound shipment

/fscmRestApi/resources/11.13.18.05/inboundShipments/{inboundShipmentsUniqID}/child/shipmentLines/{shipmentLinesUniqID}

How can I add the new line into the inbound shipment?? both front-end and REST API?