What I'm trying to do is, is the following:
When clicking on "USA" in the mainmenu and after selecting your difficulty, instead of loading a mission, the worldmap will show up like the one in challenge. From there you can select your US General and play the USA campaign. I know how to change the campaigns and link them, the problem is I don't know how to edit the menu.
Here is a visual representation of what I want to do:

[img=https://www.moddb.com/members/stanislavsaw/images/unbenannt#imagebox]menu[/img]

Any help is appreciated.

you have to edit wnd files under Window

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yes I know that. I'm talking about how to add new menus. Or editing the challenge world map in a way new campaigns can be played.

Not possible -- you can ONLY edit their positioning in the screen and edit the textures used... theres now way to add new functions for specific screen

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That really is a pity. Continue  added
new functioning bottoms, like resetting honors and opening worldbuilder from the main menu. Do you know by chance how that was done?