Unfortunately, I don't know of any easy way to do this. There is a little flash chip (IC4) which is there to provide external memory which isn't actually used at all. It uses an SPI interface - like and SD card. If you don't include this chip you might be able to hack in and SD card in its place. Connecting wires to the pads maybe - it would be a bit dicey though. You would also have to modify the firmware to read/write to the SD card.