Oh I got it!

Ok you know how each city has a 'Districts' list that you can change to the different disricts with, and one of the choices is 'International'... well make another choice called 'Free For All' or something similar, and when you enter that city in the FFA zone, and then leavee the city, you enter the outside city zone with all the other people walking around in it too and free for all pvp