Place the chicken broth, chicken breast, peas, carrots, onion, celery, garlic, thyme, basil, and sage in slow cooker. Cook on high for 4 hours or on low for 6 hours.
If you have been cooking the soup on low, change the temperature of the slow cooker to high. In a large mixing bowl, combine the butter, flour, baking powder, garlic powder, and onion powder. Mix well, cutting the butter into the dry ingredients until the mixture looks like fine crumbs. Gradually add the skim milk until a thick (and slightly sticky) batter forms. Note: the entire amount of milk might not be used or more could be needed.
Remove the chicken from the slow cooker and shred. Return it to the slow cooker and stir. Drop the batter into the hot soup by the spoon full, about 1 tablespoon of batter per dumpling. Do not make the dumplings too small or they will dissolve into the soup. Cover and cook for 20 to 30 minutes or until the dumplings are cooked through.