For the purpose of performing functions under this chapter outside the United States, the President may employ or assign individuals, or may authorize the employment or assignment of officers or employees by 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 provided for under section
402 or section 403 of the Foreign Service Act of 1980 [
22 U.S.C.
3962,
3963], or under chapter
53 of title
5, or at any other rate authorized by law, together with allowances and benefits under the Foreign Service Act of 1980 [
22 U.S.C.
3901 et seq.]. Individuals so employed or assigned shall be entitled, except to the extent that the President may specify otherwise in cases in which the period of 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 individuals appointed to the Foreign Service.