The answer is yes, but you might not want to. Overcooked chicken tends to be dry and unappetizing. The slow cooker is a forgiving environment but even so, 8 hours or more is a long time for chicken to cook. If you want to let your chicken cook longer than the recipe calls for, I recommend choosing a smaller crockpot and filling it to the 2/3 point. A fuller crockpot will cook more slowly.