Ronda, normally, to add sourdough starter to a recipe you would first remove equal amounts by weight of some of the water and flour in the recipe to compensate for the starter. But if you remove some of the mix, you don't know what other ingredients you'll also be removing and potentially creating problems down the road. If you want to use sourdough starter, I would experiment by adding a little to the mix, keeping your eye on the hydration and the dough texture. 

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