Adding potatoes is a great and inexpensive way to bulk up the soup. Again though, being that they are a root vegetable, they'll take more time to soften than say a green bean. Add the potatoes when you add the bag of frozen vegetables and broth, make sure they are cubed small, increase the simmering time for about 5 minutes or until fork tender, and also consider adding 1 cup additional broth. Use a waxy potato like a baby Yukon gold or fingerling and not a Russet or baking potato.