Go to 0x1AB200. There, you'll find a set of ten pointers. The first pointer is to the first frame's image, the second pointer to the first frame's palette (these are uncompressed, BTW), the third pointer is to the second frame's image... To add another backsprite, get five backsprite frames into your ROM and add them, in the format I specified, after the ten pointers. For more help, read the appropriate section inside the instruction manual.