I don't know if it's possible to change the behaviour of the system popups.
As a workaround to solve this item I suggest you to use a GenericMessagePopup instead of others popups. The GenericMessagePopup lets you define both the text for the buttons on the panel and the behviour of the popup (which is the active button, which responds to the escape key...). Last but not least, it gives you up to three buttons and an optional input field for the operator to write responses to the message.
All this with only a small increase in programming difficulty.
Roberto