Have not heard of such an extension. It can be done with a delimiter field, but would be static and vulnerable for anyone to place on their site. Example below of delimiter field content.
Copy below HTML to your site for certificate display:
<textarea>
<a href="http://www.example.com/index.php?option=com_comprofiler&task=userprofile&user=[user_id]">
<img src="http://www.example.com/images/badge.png" />
</a>
</textarea>
In the above you'd of course replace the domain and point the image src to a valid badge. The above would display to everyone, but you can surround it with an IF substitution to limit it to specific users or even protect it using CBSubs Fields Tabs Protection provided by CBSubs so only those subscribed could see it.