I got this recipe from this forum several years ago, which I make and keep in a jar as suggested. It has served me well.
Three-Ounce Cake Mix Upsizer
•1 1/2 cups all-purpose flour
•1 cup sugar
•2 teaspoons baking powder
•1/4 teaspoon baking soda
Whisk all dry ingredients together and store in a clean mason jar. When you wish to increase a 15.25 ounce cake mix to 18.25 ounces, add three ounces of this mix (6 tablespoons) to your existing cake mix. Believe it or not, you do not need to adjust the liquid ingredients at all — try it and see!
Happy holidays!
Gr8Auntie