There is a recipe on this site. It is a recipe to use to make a box mix taste more like homemade and also to 'extend' the batter so you have more.
This is a recipe on this site that helps makes a cake box mix yeild more cups of batter. So, if you need more than one cake box mix but not quite two, you can use this recipe. Also, it makes for a very moist and more "scratch" like cake, I love this recipe! I use it whenever possible and get rave reviews every time!
Here is the link to the recipe
If you used the extender recipe with a german chocolate box mix - would it change the flavor of the cake?