You can't, I'm afraid. Pocket WMP is limited to the codecs it
internally supports, unlike the desktop version you can't install
You could upgrade your iPAQ to Windows Mobile 2003, which will give
you WMP9, and therefore support for the Windows Media 9 codecs. That
is the only option if you must have WM9 support.
Alternatively you can reencode the files to a codec your PPC can deal
with. Use PocketDivXEncoder to convert the files to DivX/AVI format,
and play them back with TCPMP, both programs are free.