The additional damage is dealt by the same source as the original source of damage.

If damage dealt by a source you control is being divided or assigned among multiple permanents an opponent controls or among an opponent and one or more permanents they control, divide the original amount before adding the number of fire counters on Fated Firepower. For example, if you control Fated Firepower with two fire counters on it, attack with a 5/5 creature with trample, and your opponent blocks with a 2/2 creature, you can assign 2 damage to the blocker and 3 damage to the defending player. These amounts are then both modified to 4 and 5 respectively.

If another effect modifies how much damage your source would deal, including preventing some of it, the player being dealt damage or the controller of the permanent being dealt damage chooses an order in which to apply those effects. If all of the damage is prevented, Fated Firepower's effect no longer applies.