According to 5 USCS § 503, a witness is entitled to the fees and allowances allowed by statute for witnesses in the federal courts of the United States when s/he  is subpoenaed and  appears at a hearing before an agency.  According to  5 USCS § 504, an agency that conducts an adversary adjudication shall award, to a prevailing party other than the United States, fees and other expenses incurred by that party in connection with that proceeding.  However, witness fees related to subpoenas may also be controlled by specific agency statutes.[i]

[i] Carpenter v. Miller, 174 W. Va. 333 (W. Va. 1984)