Nope, no chance. I think there was someone who figured out a way to make it work. I'm pretty sure I remembered seeing the thread, but it was a lot of work and probably not worth it.
There are no adapters because the iMac's hardware just cannot act as a display. The only way to get the Mac Pro's display on to the iMac is VNC (Google it). It won't work for everything, but it can do somethings.