I would highly doubt something like this exists. This would involve a cable running out of the case from the card. In Desktops this might not be as big a problem but in laptops this would be a severe problem because the case would not close properly.
Patrick Norton from the screensavers made a cantenna for his iBook for his iBook so it is possible to hook up to regular airport. You would have to look on google to see what connetor type it is and then buy an adapter for that connector to and antenna