There is a way for direct modifications to game files to take effect, but I forgot what it was and it's still better to make a mod, because otherwise when an update comes along it will just overwrite your changes.
Mods need to be placed in
base game - Documents\My Games\GalCiv3\Mods
Crusade - Documents\My Games\GC3Crusade\Mods
Though as of a recent patch the game now looks in both folders so it's no longer that important in which you put your mods.
Just look at the folder structure of ExampleMod and take your modified Improvements file and put it in the appropriate folder.
And don't forget to enable mods in options.
