Good question.
Yes you can use a white cake mix and make a chocolate cake. Add 1/4 cup of cocoa to the cake mix and add all other ingredients as directed. I have attached another way of getting a chocolate cake using baking chocolate. Either one will work great!
Happy Baking
http://www.bettycrocker.com/recipes/black-and-white-rum-cakes/cc521dd7-82f2-4e2b-ab07-c8b0283ca39a