i'm not sure why you would want to convert an existing app into swift. There is a 100% chance of injecting new bugs into your code and a 0% benefit. All at the expense of potentially hundreds of hours of work. Its not like your app is going to be a better app just because its now in swift.
your question is pretty broad and its unclear if you are in need of setting up a leader board or getting your leader board to show up on Facebook. Those are two very different things and you would be best to tackle them one at a time.