I too was looking for this functionality.
In the end I modified the plugin so that it added the custom role(s) of my choice.
With a little knowledge it was not too difficult.
Remember you need to add the custom role to both new topic and replies I added...
Step 1 - added custom role to the arrays $newsettings_newtopic and $newsettings_newreply
Step 2 - added new case statement for the custom roles to function function notify_new_topic and function notify_new_relpy
Step 3 - Added another case statement for the custom role to function _topic_recipients_inputfield() and function _reply_recipients_inputfield()
Hope this helps!!!