It doesn't quite work like that. You create a new mod file, then use modding commands to select and apply adjustments to whatever nation you want to adjust. There's a great modding manual you'll want to check out to get a basic understanding of things, and then you're welcome to come ask at the
Dom3mods and/or
Desura Dom4 forums.
(The Dom3 forums here are effectively obsolete, and are no longer checked, really.)