I'm sure NASA will have worked out a shuttle-to-shuttle transfer technique utilizing the same mating adapters used to dock with the ISS. There will need to be some "bridge" component, as the airlock/docking adapters used to dock with the ISS don't clear the payload bay. Take a look at this photo: http://spaceflight.nasa.gov/gallery/images/shuttl
The shuttle's docking adpater extends only to just under the crew cabin. The rest of that structure is the ISS.