that depend on the script and the version you play :
_ if you play with default map scripts with vanilla/warlords , you can try
my version of some default map generator
_ if you play with a script that do not override terrain generation (most of the default and custom scripts ) , you can try to edit ..\Assets\Python\CvMapGeneratorUtil.py , "class TerrainGenerator" ... do not hesitate to ask if you want to do that .
_ if the map script override the default generation you have to edit it and look at the "def generateTerrainTypes" function
Tcho !