What you need to be a smuggler is to avoid prison.
When one side is fully aware that you are a smuggler that they can rat on to the other side whenever they want, even a child could blackmail you.
So that's one (fairly realistic imo) pathway from smuggler -> to espionage.
This is a hypothetical anyways.
MLK Jr. was being monitored by the FBI and CIA, who believed he was a Communist.