Why you'd want to, I don't know, but you can do one of two things:
- Modify the firmware of the iPod. No idea how hard this is, but it's part of the code used with the USB class when the device is connected that it identifies itself to Windows / whatever. If you know what you're doing, you should be able to rewrite the firmware to identify itself as a hard drive - there's probably some property deep down that's not worth the trouble of finding. I have a feeling this will be more difficult than it sounds as an ipod is more than a hard drive with a media player attached, it's probably only pretending to be a simple mass storage device when you plug it in. Not having ever owned an ipod I've no idea whether you can actually do this without dismantling it.
- Take the thing to pieces, take out the hard drive, and connect it to an interface circuit and plug it into your PC. This should be relatively straight forward, you could try searching for guides on replacing ipod hard drives for info.