Kek! You just described the plot of Die Hard 2. That aside it's possible, but it would take A LOT of logistical support, and any culprit would have to be physically able to access the ILS beacon on the ground. These are usually out in the open making observation possible and detection nearly guaranteed.
I would be more worried about hacking ILS through the avionics aboard the aircraft. After all, that's just a couple of lines of code and easier to plant undetected.