As someone that uses Photoshop on a daily basis, an importer would be a bad idea. The Adobe importers/exporters are bad enough (try exporting a layered Illustrator file that uses blending modes as a .PSD).
When developing a game, each sprite should be considered individually; how to minimise video memory, maximise reusability and maximise the appearance of diversity. Creating an importer would actively discourage close consideration of technical limitations.
Even so, having previously built a level editor as a Photoshop extension, you could very feasibly build a C2 exporter for Photoshop.