Magnets? Oh boy. Even the strongest magnets you could put in a football would have to be a few inches away to be attracted to a metal object. And that's if the football was stationary. To change the trajectory of an object traveling at 90 mph you would need something like an industrial-grade electromagnet. The massive kind they pick up cars with at junk yards.
I guess the goalpost could be a large electromagnet. But then you would need a significant amount of metal in the ball. And, you know, it would pull in nearby objects like headsets, watches, mess with people's pacemakers etc.