RTF is a global modification that replaces all towns in the game.

The easiest way is through the advanced map editor (you can change anything through it), but you need to figure out the special panel.

Is it the same one where you can edit battle arenas?

Added 3 minutes later
I understand now.

Added 55 seconds later
So, you need to create a map, change the city's layout, save the map, change the extension to .pak, and put it in the data folder?