No. Per diem is intended only to reimburse the attendee's subsistence expenses. You must pay conference registration fees separately, either directly or by reimbursing employees who pay such expenses and submit travel claims.
[FTR Amdt. 89, 65 FR 1327, Jan. 10, 2000. Redesignated by FTR Amdt. 2013-01, 78 FR 65212, Oct. 31, 2013]