You can! Some add-ins I like are sesame seeds, sunflower seeds, pepitas, and shredded cheese. I would not add more than 1 cup. Add them after the first round of folds, working the mix-ins in until mostly distributed. They will continue mixing in during the remaining folds and shaping. Fresh or dried herbs are a great addition as well. Add a teaspoon or two to the flour mixture at the start of the recipe.