Trovato la risposta:
//var snippetJSON= '{title:"Tin Machine II",brandnewfield:"this gets added nicely"}';
MongoDB.Bson.BsonDocument updateDoc = MongoDB.Bson.Serialization.BsonSerializer.Deserialize<BsonDocument>(snippetJSON);
var update = new UpdateDocument { { "$set", updateDoc } };
Facile quando sai come fare!
