If the new clan does date back to Enoch, then the Storyteller should come up with the answers to a few questions, unless they are willing to rewrite the history of the setting. It's also easy to replace one of the existing clans and try and fit them into the space left in the lore, so long as the new clan isn't too different from the one it's replacing.
It's easy to fudge the numbers and assume that there were other 3rd generation vampires in the city and they formed their own clans. In Vampire: The Masquerade lore, the clans were said to date back to the 3rd generation of vampires that dwelled in the city of Enoch. The current edition of Vampire: The Masquerade is still filling out its lore, so there is more room to work when creating new clans and adding them to the established history of the setting than in previous editions.
