"To be able to rotate backwards" typically requires either a freewheel or freehub, neither of which your bike has. To do it right, you'd need to replace your coaster brake hub with one of these other hub types. Replacing just the hub means you'd need to re-lace spokes to the rim and possibly replace the spokes if they're not the right length for your new hub/rim combination. It would be easier to replace the rear wheel with one that already has the hub type you want.