The Marion County Public Library did a great job of integrating a new postmodern addition to the back to their old neoclassical (Carnegie?) central library. When I vistited a couple of years ago I wondered at how easily something similar might be done at the back of Union Station (a very similar building in scale, size and style). Not many pictures here:
http://www.imcpl.org/central/index.html but I'm sure anyone who is interested could find more with a little googling.