The Barrage Generator and Overcharged Generator have the trigger of "While performing a ranged attack ...". During which step of an attack can you use this? Can I use it during step 6: "Modify Attack Dice"? That feels wrong because then the added die won't be able to be rerolled or have surges converted. I'm guessing it must be done before rolling dice, I just can't find where it states that.

You use it when you roll dice.

That's what I think, too. What I'm looking for is a reference to text in the RRG or an official ruling.

You're adding dice to the pool, so it should be used during the step 2: Form Attack Pool.

After dice are rolled there is no more pool, just dice.