Somehow there is an article for integrating OpenFeinthttp://blog.molioapp.com/2011/11/openfeint-and-admob-integrated-with.html
I would like to ask whether it is possible to integrate it into Cocos2d-x or Extension package
Yes. I did that in my free app.
Well, what I mean is whether it is possible to integrate the program source into Cocos2d-x library, or inside extension one.