A NEW face is coming to the Upper East Side’s make-believe chattering set. We hear actress Sherri Saum, formerly of HBO’s “In Treatment,” will be joining the cast of “Gossip Girl” to play the role of Holland Mercer, a powerful business executive-turned-trophy wife. “It’s safe to say she could be trouble for Lily and Rufus,” laughs a source close to the show. Saum starts shooting this week.