Bank of Marin is adding the Bank of Napa to its holdings. The bank based in Novato announced this week it will buy Bank of Napa for 51-million dollars in a deal expected to close later this year. Bank of Napa has two branches in Napa and nearly quarter of a billion dollars in assets. The deal will give Bank of Marin nearly two and-a-half billion in assets and 22 branches in five Bay Area counties.