You can add more items to the batch only in " Step 1: Confirm Inventory to Send " . If you are in any other step of the new workflow, you can cancel the shipment plan and it will get back to the Step 1 again.

You can then add the new items to the batch.

Pls Note: Cancelling the shipment plan will loose all the boxes/packing of the previous shipments and you will have to create the new shipment plans again.