Your server does not handle requests for external web pages so you cannot do any url re-writing on them. Your server just serves up an html page with the iframe pointing to some other site, then the user's browser makes a separate web request to that site to load the page in the iframe without any involvement of your server.
The IIS Url Rewriter is not involved in rendering page markup so it is not going to append anything to the url for the iframe, it only routes requests that come to your server.
Hope it helps,
Joe