Yes, you can use activated sourdough starter in our recipes. My own sourdough starter (see link), after I activate it from the fridge, is about half water and half flour (you can find recipes for naturally-fermented sourdough starter all over the web, and one of our future books may have a recipe of our own). I’ve found that about 1 1/2 cups of activated sourdough starter works well in our full-batch recipes, which make 4 to 5 pounds of dough. This means that you need to decrease the water in the recipes by 3/4 cup, and the flour by 3/4 cup. At the end, you’ll probably need to adjust water and flour to create a dough that looks and feels just like what you get with our yeast-based recipes.
So, having done this, do you need to use commercial yeast in addition? I found that I still needed some yeast in the recipe, though I could use a lower dose, which I’ve posted about before in the context of our yeast-risen recipes. That seems like a good compromise. I did experiment with zero-yeast versions, but I found them a bit temperamental– didn’t store terribly well so we decided not to put that in our books… yet!
More in The New Artisan Bread in Five Minutes a Day, and my other books.