As a security principle; You are not allowed to do anything in an iframe that's base uri is different from what is in the address bar; You just allowed to manipulate the iframe'e that's src tag is equal to page's.
Think about what would be happen if you insert a bank's internet banking in an iframe and cheate the people and in backend change the source of money transfers to your own account :D
but what you can do is to get the html source code of the where you want to show in an iframe; and show it using javascript; then you will be able to treat with that source code like your own.