Yes, it is possible to use regular cake mixes in an Easy Bake oven. To use a regular cake mix, combine 3 tablespoons of any boxed cake mix with 3 tablespoons of water or milk.
Mix until the batter is smooth, and bake 10 to 15 minutes in the Easy Bake oven. Alternatively, use traditional recipes to bake from scratch, and adjust all amounts downward so that the batter fits in the toy-sized pan. For instance, an Easy Bake chocolate cake recipe includes six tablespoons each milk, flour and sugar, 1/4 teaspoon baking powder, 1 teaspoon cocoa, 3/4 teaspoon shortening and a pinch of salt.