If you run a trace, you should see the method that is called (Likely SalesOrder.CopyOrder).
Then in a Method Directive BPM, you would use this method to set your values.
You may need a trace to see the steps when you set the prices to zero too.
It would likely be in these steps on Post Processing:
SalesOrder.GetByID widget with the new SO#
Run the same methods and changes from your second trace. (Use the Epicor Trace Differ tool, its kinda awesome).