Except that is not what nomadic lifestyle is. Almost all nomads have a fixed location or migration path that does not vary much from year to year. In some cases this path may be within just a few kilometers, others over hundreds of kilometers. The only times it does move out of this range is when the environment changes which includes over population.
This is why I believe it could be handled with just a new type of "city" screen and new "settler" unit.
NOTE - need to remember that the code for building settlements using a settler does not keep the information about the settler where the AI is concerned; so some other way is needed to pass the information about the "settler" equipment and knowledge, to the new "city".