Just going in blind, but it will require some DBC edits. I'll find all the files I can think of that pertain to arenas.
AreaTable.dbc Specifcally column 5 and the proper flag. BattlemasterList.dbc
Possibly map.dbc
along with those DBCs i'm sure you will need some core edits to go along with it. If i think of any more DBCs i'll update this list.