If you don't have cooked chicken, you can use raw chicken. Cut it into small pieces, then add it to the vegetables when you saute them. Stir to ensure the outsides are white, and the rest will cook as you cook the soup.
If you want to make this vegetarian, skip the chicken entirely. Use vegetable stock instead of chicken stock, and add extra veggies to bulk it up.
When you reheat the soup, do so gently. In a stove over low heat is best to avoid the soup separating if you accidentally heat it too high.
For more tips and tricks, be sure to read the full article above.