Add classes to advlink plugin?

Hi,

I think I could be missing something really simple here.

I'm trying to figure out how to add options to the class drop down in the pop up window for adding a link.

I can add classes in the system settings for tinyMCE as a whole, and that gets carried over to the list in advlink, but I just want them to show up in advlink. I would like to do the same for advimage.

I see that the settings to change are added to "advlink_styles"http://www.tinymce.com/wiki.php/Plugin:advlink
but I'm not sure where or how to go about adding that to modx. Simply creating a system setting doesn't do it. (but I might be doing that wrong too)

Thanks in advance