yep, WDW has thought of all the possible ways people could try to get around the FastPass system, and no, you now cannot use your extra tickets to get FastPasses.
you must use the tickets you used to enter the park that day.
expired tickets won't work, and neither will valid tickets you didn't use that day.
Disneyland has an interesting way of using FastPasses, i believe it's slightly different from WDW, and better because i remember being able to get a 2nd FastPass if your first FastPass is scheduled to be over 4 hours away from the current time. i don't think WDW does that though it'd be great if they adopted that as well.