Here you can find some info about iRiver firmware hacking.

Our people found how to unpack firmware file and how to pack it after hacking. Also they found how to convert Iriver iMP-100 firmware to AVC Soul format. (Last Soul firmware was released at the beginning of 2001, while new firmware versions for Iriver appear permanently.)

Here you can find ready hacked firmwares and tools for hacking.

Note: using unofficial firmware can damage your device! Use it only if you are sure what you are doing.