I would not. If you have leftover chili mac I would take it out of your pot and transfer it in to a bowl. Allow it to cool completely before covering and storing it in the fridge. Pasta can turn too soggy and soft when kept on warm in your slow cooker for any length of time. If you're just cooking noodles by themselves in a slow cooker, add a small amount of oil and liquid (preferably sauce, but water will do). You can keep pasta warm and moist without overcooking this way.