Additional Levels to Existing Enchantments

This can be done by making custom enchantments simply labeled (sharpness 6 (or VI)) and have it do extra damage through procedures. 

However, I would simply use the built in enchantment system, as you can go above sharpness 5 for example, with commands. You could have a special block for instance that enchants any item with a more powerful level when right clicked by using the execute command procedure. 

Commands are really confusing, so I'd recommend something like a command generator to help: https://www.gamergeeks.net/ 

This isn't a perfect solution but it might put you on the right track.