Any town, city or borough, in addition to such powers as it has under the provisions of the general statutes or any special act, shall have the power to provide by ordinance for the appointment or election of alternate members to its zoning commission or combined planning and zoning commission. Such alternate members shall, when seated as herein provided, have all the powers and duties set forth in the general statutes or any special act relating to such municipality for such commission and its members. Such alternate members shall be electors and shall not be members of the zoning board of appeals or planning commission. Such ordinance shall provide for the manner of designating alternates to act.