I don't know if this is the one on the site but I use it all the time!!
1 cup flour
3/4 cup sugar
1 tsp baking powder
2/3 water
1/2 cup vegatable oil
add this to every box mix.Should yield an exrta cup cup and a half of batter.
1 cake mix
1 cup flour
1 cup sugar
pinch salt
3 eggs
1 cup melted butter/marg
1 cup sour cream
1 tsp vanilla
1/4 cup milk
1/3 cup water
* if making chocolate cake add 3Tbsp cocoa powder
This is the one I use all the time and everyone raves over it.
Melissa 
I tried the cake extender recipe just adding flour and sugar. For me it was too bland. The extra flour diluted the cakemix flavor. I'd try Melissa's. The added butter should help the flavor. I would probably double the amount of extract since that's a lot of batter for 1tsp.
You pretty much make a scratch cake. I have heard of adding sourcream to scratch cakes to make them more moist, but the cakemix is already moist. Do you always add the sourcream? TIA
Whether you have to extend the cooking time is going to depend on the size of the pan you are using. You don't want to go much more than half full on your cake pans. If you add too much batter, it will end up all over the bottom of your oven.
I use the extender when I'm making a larger sheet cake or say a 12 inch round.
hope that helps
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