The judges of the United States Court of Federal Claims may appoint necessary law clerks and secretaries, in such numbers as the Judicial Conference of the United States may approve for district judges, subject to any limitation of the aggregate salaries of such employees which may be imposed by law. A law clerk appointed under this section shall be exempt from the provisions of subchapter I of chapter 63 of title 5, unless specifically included by the appointing judge or by local rule of court.