You'll add the stuffing to the turkey halfway through the cook time. So the turkey will cook on its own for 3 hours; you'll add the stuffing, then it will cook for the final 3 hours.  If the stuffing were cooked for the full 6 hours with the turkey, it would dry out. If you opt to cook the stuffing outside of the turkey, place it in a shallow baking dish and cook it in the pellet grill for 3 hours to heat through.