We have tested this quite a bit while developing recipes for our cookbooks and had varying results. The best method is to cook them separately and stir in at the very end. However, if you want to add them directly to the slow cooker, it's best to add them towards the end of the cook time. Keep an eye on when it is tender (it really depends on the size of the pasta or the type of rice). Make sure you have plenty of liquid in the soup, as the pasta or rice will absorb some of it.