How can you change flavors in store-bought frosting?
Yes, you can certainly mix in something to change the flavor, and cream cheese sounds like a good option.  You will probably need to soften the cream cheese (microwave for a bit, but don't melt it), whip it, and then add in the store-bought frosting and whip until fully incorporated.  Depending on how much cream cheese flavor you like, I'd recommend something like 4 oz. per can of frosting.