Unfortunately, you can't modify the existing buildings without over-writing the entire definitions file in your mod (the "total conversion" method). It can be done, sure, but it will not be compatible with other mods that change the default buildings. Such is often the case with balance mods. As long as you're the only one who will be using the mod, this won't be a problem.
The file is in data\Game\ImprovementDefs.xml
