So in Heroes of the Street the seigebreaker archetype for fighters grants this ability.
Breaker Rush (Ex): At 1st level, a siegebreaker can attempt bull rush or overrun combat maneuvers without provoking attacks of opportunity. When he performs either combat maneuver, he deals an amount of bludgeoning damage equal to his Strength bonus (minimum 1). If he has Improved Bull Rush or Improved Overrun, the damage dealt by the appropriate maneuver increases by 2 and he adds any enhancement bonus from his armor or shield (though such enhancement bonuses do not stack, if both armor and shield are magic). This ability replaces the feat gained at 1st level.
Can I combine it with Power Attack to deal more damage?
Can I wear spiked armor to deal more damage when I smash into people?
Power Attack only applies to attack rolls so no. You only add what it says you do. Same goes for armor spikes, unless armor spikes have something they say about Bullrush or Overrun anyways.
Power Attack penalties apply to both attack rolls and cmb rolls.
What happens if you have improved bullruah and overrun both does this add a +2 for both or only the +2 when using a bullrush if you have improved bullrush?