The enabled / disabled state of the buttons is controlled internally by a logical check whether the source collection allows or not adding /deleting of items.
So as a workaround I may suggest to handle the Beginning Edit event. Inside the event you may hide the Add button , and after edit - show it again .
For this you will need to use the CommandButtonsVisibility property.
In case you have troubles implementing this , just let me know what kind of source collection do you use , and I will prepare a small sample for you.
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