I think this is not so easy if it was not designed by the manufacturer to suport these formats. I do not know the exact details about how a DVD player works, but if the decompression of the video stream is done by hardware decompressor, then you should change the CPU...
If the decompression is done only by software then you should obtain a firmware which has the decompressors in it for divx and mpg . I think this is your only chance.
Perhaps the simplest sollution is to buy one divx and mpg ready, they are really cheap.
/pisoiu