Add post tags to previously created custom post type

Question

I am trying to add post tags to the custom post types bbpress creates, so that I can do some "related topics" kind of stuff.

Is there a function that adds a post support like 'taxonomies' => array('post_tag'), but to a post type that was previously created?

My thought was I could just re create the custom post type on theme install or soemthing and hope it didnt mess anything up, but I really wanted to avoid doing that.