How do I add verticle posts to an existing deck railing to support a pergola? The current deck railing is constructed of 4x4 green treated posts capped with 2x6 cedar rails. Do I have to add additional posts next to the existing posts to support the pergola or can I chisle thru the cedar rail and somehow join the posts together end to end? Deck size is 12x18 with post spacing about 4 to 5 feet,

Thanks, Vic