league-of-legends – How to Watch Pro/Tournament Matches from Within League of Legends

It is not a game feature nor a Hack.

The 6th player is a Mod/GM that can only be added by Riot staff; this player has the ability to see the map without fog of war and it is also capable off seeing both side stealth units.

Unfortunately I dont have a source to back up my statement, you can check on any of the matches where a 6th player is, at least one of them will have the name of a Riot employee, most likely Phreak and the game type play is a Draft Pick Custom Game.

You can customize the UI and use addons. One place you can look for addons is here, and this blog talks about an officially supported mod that allows you to change your UI, among other things.