CHICAGO, Feb 20 (Reuters) - United Airlines on Wednesday said it would add more locking devices to the cockpit doors on 93 Boeing (BA) 777 and 767 jets following an incident two weeks ago aboard a Miami-to-Buenos Aires flight in which a passenger crawled partially through the door. On Feb. 7, a passenger on Flight 855 was able to get a small portion of his body through a 10-inch bottom panel of a cockpit door on a Boeing 777. The United Airlines pilot hit him with an ax located in the cockpit to subdue him. United, a unit of UAL Corp. (UAL) and the No. 2 U.S. airline, said its plan calls for the upper and lower portions of the cockpit door frames on the 93 planes to be reinforced with a locking capability that will make the door impenetrable. The doors on all United planes are already reinforced by special metal locking bars. The airline said the the rest of its fleet of about 543 planes have different cockpit doors that do not require additional strengthening.