Males do not take family names, as they are each considered the ‘origins’ of new families, but while not impossible, a Seeker of the Sun choosing his mother's name to be his last, would amount to shunning his culture and adopting the rules of the Keepers of the Moon. There will be some Seekers of the Sun who will respect this, but others who frown upon it.
There are tia within the twenty-six existing tribes who, instead of defeating an existing nunh, prove their worth to the tribe by extending its hunting grounds. These tia will venture out into the world and claim territory of their own (by either finding somewhere unoccupied, or taking an area by force from another tribe). If they can maintain it for an extended period of time, then they become the nunh of that area, while still remaining a member of their original tribe. This actually happens quite often. And is far more accepted than merely leaving the tribe to make one's own (which, ultimately makes that person an outcast). This is why you will almost never see Seekers of the Sun with tribal letters beyond the original twenty-six. There are simply not that many, and those that do exist, rarely admit it, for fear of ostracism.