No idea how to do that.
Do i have to export?
Or is it simply changing it to xml when I save it?
You should edit the xml file. There is no way currently of saving as an XML file (but Wisconsin is working on it). As I understand it, all your mod does is change the setup. That is easy to by directly editing the XML code.
The procedure to create new mod from an existing file is.
1. Unzip the existing file to a folder within the maps folder.
2. Change the name of the folder to new mod name
3. Open the xml file (you can used Wordpad, Notepad, Crimson editor or any other text editor. Don't use Word.
4. Find the line that says "Info Name=". Change it to match your mod name
5. Find the "ownerInitialize" section. Change the territory owners as required
6. Find the "unitInitialize" section. Change setup as required.
7. Find "Map Name=". Follow it with the name of the new mod
8. Find property name="notes". Put your own notes. Make sure to end every line with <br> 9. Save and test the file. There are sure to be spelling errors.
10. Zip the contents of your mod folder up (use 7-Zip).
11. Place the Zip file in your maps folder
12. Deleted the unzipped folder (duplicate entries mess up TripleA)
13. Let the world know of your new mod.
This is a bit more work than just using the edit mode, but there are advantages.
1. It will work even the player does not have the mod you based it on.
2. Other designers can edit your mod
3. It is more likely to work after the next update
4. You can tweak the graphics files
5. You can put notes in to explain your scenario and house rules.
6. You can change functional rules. For example "Do not attack neutrals. " could be done by just adding the NeutralsAreImpassable property and setting it to true.