It's possible to add a new gens, but it's complicated enough that I don't have enough time to tell you how. What I'd suggest is that you use one of the existing gens and family name traits (GensOctavia and Octavivs would be best) and make the following text substitutions in text\export_VnVs.txt, MAKING A BACKUP COPY OF THE FILE BEFORE EDITING IT:

{Gens_Octavia} Gens Vitellia

{Gens_Octavia_desc}
(Interesting information about Gens Vitellia)

{Adopted_By_Gens_Octavia} Adopted into Gens Vitellia

{Adopted_By_Gens_Octavia_desc}
This man has been adopted into Gens Vitellia.

{Adopted_By_Gens_Octavia_effects_desc}
He is treated as if he had been born to the Vitellii.

{Octavivs} Vitellivs


Be careful to preserve existing tabs in this file. That should do it.