I went to a local craft brewery (Long Ireland) the other night and was chatting with the brewmasters. They have a Breakfast Stout that is out of this world. I tasted heavy coffee notes and asked how they did it. They added Kenyan coffee beans to the secondary. It is a cream stout with coffee, 3.75% ABV and 30 IBU's.
They also told me that they make pancakes with it instead of water. I made waffles instead. Good stuff!
You can add flavor to a porter or a stout BB Kit by adding more chocolate malt or roasted barley, you can add/change hops or use a different yeast. But at this point you might as well create your own extract recipe from scratch or brew a new extract recipe from a book, magazine or the internet.