How to set up bike so that back-pedalling stops the rear wheel?
This is called a coaster brake. To install one of these, you'd need to replace the rear hub, and remove the existing rear brakes, which sometimes means replacing the rear wheel, and there may be other complications. I recommend retaining your current front brake as a backup; if the chain snaps, you lose your rear brake. (That's happened to me exactly once over the years, and that bike had handbrakes, not a coaster brake.) 

While Shimano makes an internal hub with seven speeds and a coaster brake, I'm not sure if their 8-speed hub has one. 

Can you tell us more about your current bike? Does it currently have gears in an internal hub on the rear wheel, or is the gearing via conventional derailer gears?