No, the FLASH ROM cannot be expanded. You would need to change the Flash ROM chip and also the operating system so that it would be able to use the bigger ROM.
The disk direct function is the substitute Yamaha has provided for the additional Flash ROM. It is pretty fast, but naturally cannot match the speed of the Flash. Another drawback is that you cannot use the One Touch Settings associated with the style (if any). I'm hoping (like some others) that Yamaha would allow loading of the OTS settings directly from disk in a future OS upgrade.
Hope this clarifies the question.