[{"Id":1,"extension":[{"key":"Name","value":"Bob"},{"key":"Age","value":20}]},{"Id":2,"extension":[{"key":"Name","value":"Sam"}]},{"Id":3,"extension":[{"key":"Age","value":25}]},{"Id":4}]

db.collection.update({"extension.key":{$ne:"Name"}},{$push:{"extension":{"key":"Name","value":"Default"}}},{multi:true})