Well, there's the default pretenders. AFAIK you can't select or de-select the default pretenders.
But if you made a commander you wish to make a pretender, use;
#pathcost XXX
#startdom XXX
which will make your commander/unit available as pretender.
But I made a request for something that might help to do so;
http://forum.shrapnelgames.com/showthread.php?t=41813