I know that buying a chocolate cake mix would be the easiest, but there is a white cake mix whose flavor I like...
What may be easier is to get unsweetened chocolate (Baker's is the most common brand around here). Melt it (I do it in the microwave) and stir it into the batter once it's made. Minor stirring will make it marbled, and major stirring will give you chocolate cake. This can solve the problem of throwing off the wet/dry ratio Michael mentioned.
In a scratch white cake recipe, you could replace part of the flour with an equal amount of unsweetened cocoa. However, that's not possible with a cake mix, which already has the sugar and flour mixed together.
I would recommend either adding a small amount of melted unsweetened chocolate (no more than 2oz) or you could add chocolate extract.