Unfortunately, you cannot directly extend a repeating meeting series by simply adding new dates to the existing original schedule. The best way to add new dates is to create a separate appointment/meeting for it. There is already an existing enhancement request for this feature which you can help up vote via this link.
Since it is not possible to extend a repeating meeting, the easiest way to create a new meeting based on an existing one is to use the “Copy Into New > Calendar Entry” option from the Action menu.