you can edit units changing their xml files or their tga files, xml for behavior and attributes and tga for texture, they have also gd3 files which are their models(shape), upgrades are editable in xml files, tilesets can be created using xml, gd3 and tga files (config, shape and texture), the game can be translated using language files, races can be configured grouping units and upgrades, which are also configured by xml, maps can be created using the map editor, and scenarios can be created using xml, in fact everything in the game uses xml for configuration, other than that you'd have to change the source, which is opensource and downloadable in the forums.
that's all
