Yeah, you can do that.
The files needed are:
custom_locations in the data folder.
And for some battles(usually those that don`t let you create a settlement, like the sarmatian mound and grassy flatland in rtw vanilla) you`d need to edit the file descr_battle from each sub-folder in Rome - Total War\data\world\maps\custom. I think these have a custom made tile so that`s why they`re in a separate location. But these might only be used in RTW and not BI. Better check first.
If you open any of the files you`ll see some coordinates:
Code:
custom_location British Grassland
{
location 123 96
image data/menu/grass1.tga
sett_locked no
summer no
}
The blue is the x value and the y is the green. And you can find other coordinates in game by loading the campaign and using the show_cursorstat command.