Cross-domain and iFrames

Question

I am trying to something I really can't figure out. I have an iFrame loading some content from another domain on which I have no power. What I am "simply" trying to do is fetch the content of the iFrame to use it in PHP/Javascript. This is where I faced the "cross-domain" problem. I am unable to access the content of the iFrame. So frustrating.

So I read, and read some more and seen about the "postMessage()" function provided by HTML5. I saw a solution with this system, but I still can't figure one point. Basically, it works with a sender and a listener. So I need to have a listener in the iframe that, when triggered, will send the content back to the main window.

But HOW do I add some code in the already loaded iFrame without deleting the content ?

I don't really need to use postMessage(), I can be anything as long as I can get this damn content !

Any suggestion is appreciated !

Thank you !