For the purpose of performing functions under this chapter outside the United States, the President may employ or assign persons, or authorize the employment or assignment of officers or employees of agencies of the United States Government which are not authorized to utilize the Foreign Service personnel system, who shall receive compensation at any of the rates established under section 402 or 403 of the Foreign Service Act of 1980 [
22 U.S.C. 3962, 3963], together with allowances and benefits thereunder; and persons so employed or assigned shall be entitled, except to the extent that the President may specify otherwise in cases in which the period of the employment or assignment exceeds thirty months, to the same benefits as are provided by section 310 of that Act [
22 U.S.C. 3950] for persons appointed to the Foreign Service.