I was wondering if I can add more flour, sugar (maybe another 1/2 cup or a whole one of each ) maybe another egg and extract? some times I need a little more batter but not enough to make an extra whole receipe.
Any experince or sugestions?

I make a half a box if I need a little extra. I simply weigh out half the cake mix and then cut the extender ingredients in half. Careful, though. The weight on the box is NOT always the actual weight of the mix inside the box. Weird, huh?

Different extender recipes, as well as different cake mix brands yield different amounts of batter.

The WASC cake made using DH white cake mixes yields a tad over 14 cups of batter.

Here's the expanded flavors version of the recipe:

http://tinyurl.com/2cu8s4

HTH