It's plausible to have an American representative allowed to know some details of a case, such as a member of the military if the deceased was currently serving, or a member of the American embassy, but the Met would never allow an entire team of investigators to solve a case for them, the implausibility of the premise hurts the fic, I'm afraid.

You could possibly have a member of NCIS team who is in London, who knows the victim intimately (meaning next of kin) who sees that Sherlock has access to the case and they team up with him, (in which case they would have to bring something to the investigation that was necessary, information that no one in London could know) and you might be able to pull in one other member of the team, that would be an interesting foil to the Holmes/Watson pair.