How to change workflow for Shipment

I want to change the workflow on shipment and add a Button “READYFORPICKUP” in between Confirm shipment and prepare invoice.

When CONFIRM SHIPMENT is clicked, A new Button (status) should appear,

Once this button is clicked, then it should return to normal workflow and Prepare Invoice should be available.   Prepare Invoice should not be available until Ready for Pickup has been clicked.  I have tried to build this in the workflow, with mixed results.

What is the best way to accomplish this.

thanks