Add a new Rule to the Ruleset.  Call it “Clans” and give it the following text:

Each Clansman is a member of exactly one Clan.  This information is tracked in the GNDT.  New Clansmen are in Clan Null.

A Clan Lord who is not the Patriarch and who is in the same Clan as another Clan Lord may change their Clan to any proper noun of 10 letters or less that is not the name of a currently-existing Clan.  This represents founding a new Clan.

Any Clansman who is in Clan Null may change their Clan to match the Clan of any Clan Lord.

Set the Clan of all Clansmen to Null.