Not sure why someone would think an expedition couldn’t be stolen. If a thief is motivated enough, your vehicle will become theirs. Anything you want to add would just be another deterrent on top of the factory theft deterrents already in the vehicle.
You can place a master switch on the power cable running to the fuel pump. No fuel no start. Would be a good deterrent but doesn’t guarantee that a chop shop won’t employ a flat decked wrecker and slide blocks to take it…