You can add custom fonts but need to jailbreak the device first, although others can correct me but maybe the old non-touch Kindle's can use the USE_ALT_FONTS hack without jailbreaking?
You should check out the developers forum for help with this. The jailbreak thread for a Kindle 4 is -
Font, ScreenSaver & USBNetwork Hacks for Kindle 2.x, 3.x & 4.x