How to turn a brownie mix into a cake?
Adding one more egg will give you more "cake like" brownies. 

If you have baking soda you could use that as leavening. If it were me I would add 1/4 tsp. baking soda to the dry mix and stir to combine. I would add additional liquid in the form of 1/4 cup of buttermilk or soured milk to activate the soda and use an extra egg. This should give you something that is more cake like, than brownie like.