You could set up Click event handlers for the buttons that enable/disable
the validators as necessary. I'm not sure how this would work within the
event cycle. You may have to mess around a bit to get it working.

// Pseudo code
OnAddClick
{
  Disable SubmitValidator
  Enable AddValidator
}
OnSubmitClick
{
  Disable AddValidator
  Enable SubmitValidator
}

To make your interface more humane, you will probably want to try to run the
"Add" code as well. Otherwise, data may be lost if they enter the last item
and click "Submit" instead of "Add".