Is there an add-on that can do this?

[ipunkbali]

Basically the Wordpress comment-link is directed to forum post when user click the link.

Example: breatheheavy.com

 

[LPH]

You will want to look in the resources, bridges:

http://xenforo.com/community/resources/categories/bridges-integrations.17/

The link itself is simply a WordPress theme change.