Can you preserve leftovers by canning them in your canner?

You should not in most cases. Canning foods is really a science of combining specific amounts of ingredients to achieve a product that meets a necessary ph level required to preserve the food. If the ph level is not acidic enough the food will develop botulism and can kill you. It is not always easy to detect the presence of botulism in food so following recipes that have been tested in kitchens approved by the USDA is very important. Please do not experiment with canning foods.