I am fairly new to modding myself, but as far as i understand that modding of certain things in Divinity isn't done in the editor, its done in the source files for the game. you have to get an extract tool to extrack the package files, then edit them from there.
Once you have a saved mod from the editor, copy your source files into the saved mod location (should be in the same place you have the game itself saved labeled as a mod folder), then enter editor again and publish. It will publish to your computer under Documents>Larian Studios. In the editor you can add new weapons, items, levels, etc. If you can do more than that, i havent found out how yet.