I forget,

[chrispedersen]

When you are creating a new nation, how do you select which pretenders are available to the nation?

[Thilock_Dominus]

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