I seem to recall the Input module in Ace to only recognize specific button presses, and even then, they are, for the most part, attributed to the virtual controller.
Saying that, the "Shift" button is, by default, attributed to pressing the virtual button "A", if memory serves. So, you could do it through event-commands. Such as a Conditional Branch checking if "A" has been pressed.
*Edit: My mistake. Shift is a keypress that the Input module recognizes. I'm seeing an Input.press?(<
symbol>) method in the help-file, though. So, it would probably look more like...
...so.
*Edit2: Confirmed.