Do you think it's possible to create a sleeper agent like in spy movies?

Yes

They have researched psychic behavior (Stargate, Monroe Institute), psychological profiles ( I don't recall, but Rommel or Hitler was the first target), and elements of mind control.

If someone that has suffered a traumatic event and gets triggered by certain words or behaviors, it's not unreasonable to think that someone might be manipulated with a code word or something similar, if they were under hypnosis or some other kind of suggestive mood.