If I'm not mistaken, Ultegra 6800 cranksets are designed to be compatible with standard (narrow) BB shells, so, you can get an adapter to put a 6800 crankset into a BB30 frame (standard shell is 33-35 mm in diameter, BB30 is 42 mm, some others are even bigger), but you can't put a BB30 crankset into a standard frame. Many cranksets come with several installation options - you just pick the right bearings and spacers and you can install the same crankset into different frames.