No federal statutes generally authorize foreign law enforcement agents to carry firearms in the United States. In particular, 18 U.S.C. § 951 does not provide such authority.
Absent congressional consent, the Emoluments Clause precludes foreign agents from enforcing federal laws. 19 U.S.C. § 1401(i) does not constitute such consent.
The President does not possess inherent authority to designate foreign agents to carry firearms in the United States in order to enforce federal law.