why don't you just use your smart media card slot to do this, so easy to do. make a folder called CASIO_MD and put your midi files in it. seems that they are ordered by date. oldest first, therefore adding new songs doesn't change the order.

as midi files are so small the 8MB cards are ideal, so if you know anyone with a digital camera that using smart media cards they are likely to have a small meg size card, eg 8 or 16MB they don't use as they are really too small for camera use but ideal for Casio keyboards that have smart media slots. lucky for me i had just one of these with my old fuji a101 camera.