It depends what the banner is and what it does. You can put an image of the banner on your hub as an image but not a click through advertising banner. If you want to go one further you can put a text link with it saying something like "go take a look at what WWF is doing, or perhaps a small news item from theree. Then people can click through your text link.
Does anyone use them on HubPages? If so then also, how do you add them because the code is generated as HTML and last I knew we can't access the Source code. Is there a way to add into a photo or link capsule?