You cannot 'add' internal GB other than by replacing the HD, an option that I am considering now that my AppleCare has nearly expired. If you decide to go this route get the largest HD that you can afford. The iBook takes 2.5" (P)ATA/IDE interface, the max currently available, I think, is 160GB.
DIY fitting instructions can be found at iFixit.com but not an enterprise for the fainthearted.