How can I fix sunken couch cushions (PS, they can't be removed or opened)?

The couch is "leather" and I just bought it off of a friend. For the price she gave me, I didn't think I'd mind the sunken cushions... but I do. So, there's a huge dent in the middle of the couch. The cushions are attached to the couch and can't be removed - so I can't put wood underneath them. There's no way to open them, either - so they can't be re-stuffed. Any ideas?