I don't think even a LEO from elsewhere can carry a handgun in NY or at least in NYC. Even NY State Police are banned from entering the city with guns, as are any LE officers from NY state or anywhere else. And the NYPD has made it clear that they will NOT accept any Federal law to the contrary; they will make an arrest, jail the subject and let the "suits" worry about it.
A couple of years ago I spoke with a police sergeant from Kansas City, MO, who flew to NYC to pick up an extradicted prisoner. When he got off the plane at JFK, he was met by an NYPD Lieutenant, who made him open his suitcase right there and confiscated his gun. He was told that no one, NO ONE, was allowed to possess or carry a handgun in NYC except a permit holder or an NYC or federal officer. That did not include LEOs from anywhere else, even NY state, no matter why they were in the city.
When the prisoner was ready, he was taken on board the plane by NYPD officers, who then and only then returned the MO officer's handgun.