I want to try the "cake mix extender" w/ chocolate cake. I think I read here that you need to add some other stuff, but now I can't find it. Can anyone helpl?

I don't know what recipe you're using, but the one I have is the same as any other flavor, only you add 3 Tbsp cocoa powder also.

You might try and modify this with a little cocoa instead of a full cup of flour. I've tried this one and it was great. Turned one mix into about 1 1/2.
http://www.cakecentral.com/cake_recipe-1977-22-Cake-Mix-extender.html