It's a bit like doing overlays in web pages... you need to create the overlay and put it in your MCML, but make visible="false" on the element.
Then, when you want to display the item, set visible="true" to bring it up. Put a NavigateInto in your overlay UI to get focus on the first item on the overlay, and put a ColorFill that captures mouse clicks behind the overlay to stop the page behind from being focused.
Cheers,
Andrew