I couldn't add something like that to this pack because it would turn Wikipedia URLs into huge iframes in the middle of a post. That seems like something that would be better handled as a custom BBCode anyway.
Try creating a custom BBCode as described below, but I recommend you ask in the support forums whether it could pose a security issue to use {text} inside of a URL. And please tag me if you do so I get to read the official answer. From what I can tell, {text} is automatically URL-encoded so it shouldn't be
a problem, but I'm not familiar with XenForo's inner workings so I don't know how it really works.
Code:
BB Code Tag: wiki
HTML Replacement: <iframe src="http://en.wikipedia.org/w/index.php?title={text}&printable=yes" width="500" height="500" frameborder="0"></iframe>
Example Usage: [wiki]Occam's razor[/wiki]