it probably wouldn't be as safe as a bike helmet.
bike helmets are designed for one time, heavy impacts, like seen in bike accidents. they're designed to split and crack when your head hits the ground or a car, instead of your head splitting.
football helmets are designed to protect repeated, lower impacts (not much lower though
) they're designed to protect a player from neck injuries and concussions. (and recent studies with ex-football players tendencies of depression, suicide, and early onset alzheimers shows they're not very good at even doing that)
helmets are designed, and only work correctly for a specific activity.