I have attached a product that should do what you are asking. Once you import the product, go to its settings and enable it, and enter the list of usergroupids whose primary members will be added to the forum moderator lists.
For example, if I wanted admins and supermods to be added to the list, with admins first, then supermods, I would enter:
6,5
into the usergroups settings. Individual users will be sorted in ascending order by userid within each group.
Please let me know if there are any problems or features you would like to be added.