Is this even possible?
DivX is MPEG4. I am not sure I undrestand your question.
You can have MPEG without DivX. They are not the same thing.
www.vcdhelp.com will have information on converting. I prefer TMPGEnc.
There may be a better solution, depending on why you want to convert. You can decode the DivX (MPEG 4) and re-encode it as MPEG 1 or 2, but it’ll take all night, and you’ll either lose a lot of quality or gain a lot of file size.
For example, if you just want to encode a DivX movie as a VCD so you can watch it on your TV, you might want to get a TV-out card instead.
TMPG can do it, at www.tmpgenc.com. However, to get the same quality, you need to go MPEG-2, and the file size will be HUGE! I took at 600 meg Divx file, TMPG gave me an estimated size of 6 GB.