It's not advisable to mix raw with dog food with grains because it will confuse the digestive system. If you feed grain free food, you can add some meat to the dog food or you can do 2 meals, one with dog food and one with raw. I add raw to Titan's food and he is fed twice a day.
In the morning, he eats 1/2 cup of dog food with scrambled eggs with crushed glucosamine and probiotic and 2 small pieces of meat. In the evening, he eats 1/2 cup of dog food and a pound of meat. Whatever dog food he doesn't eat at either meal, I add it to the next meal.