Howto edit the Xcode project.pbxproj "SystemCapabilities" using Xcode manipulation API?

[Solved] I would like to add 2 capabilities to the generated Xcode project file in a postbuild step using the Unity Xcode manipulation API. More precisely I want the string

SystemCapabilities = {
com.apple.GameControllers.appletvos = {
enabled = 1;
};
};

to be converted to

SystemCapabilities = {
com.apple.GameCenter = {
enabled = 1;
};
com.apple.GameControllers.appletvos = {
enabled = 1;
};...

Howto edit the Xcode project.pbxproj "SystemCapabilities" using Xcode manipulation API?