It depends. If you simply mean as a prop or item, using various shapes ands stickers will help you make one. If you are referring to a level key, those are obtained when you publish a level. They are in your tool bag, on the page with "Basic Kit", "Enhancements", "Dangerous", etc. (the third page, also titled at the top of the pop-it as "Gameplay kit"
you cant....
You can add keys, but in some levels you can collect a key which unlocks an extra level.
You cannot.
use an emitter
you cant
With a scoreboard
Make a good level
Make a really good level
Make a good level
Because you can make your own levels
Use options to make it a favorite
You just go to create mode and make anything