V8 Process Security

Further to my discussion on field security I have come across process
security. This allows me to apply security settings on some methods in
a particular process. The problem is that in the drop down list of
available methods, the one i need is not there.

The method I would like is AllowReadyToInvoice in the Customer
shipment, there is a CheckReadyToInvoice but this method doesnt do
what we want. Does anybody know how to add new methods to this process?

In other words i would like to be able to stop users doing a
particular task (Ready to invoice) unless they are given permission to
do so.