Sometimes there is a perception that a succession plan can only include current, internal employees. While it is certainly a goal of succession planning to increase retention and career opportunities for the great talent you already have in the organization,
the ultimate goal is to reduce talent risk management for the organization. So, if a manager knows of a good external candidate who should be considered if the position becomes vacant, then it is perfectly fine to put that person's name on the succession plan as a potential external candidate for the position. Obviously, all external and internal job candidates must go through the interviewing process if the position becomes vacant, whether or not they are on a succession plan.
The other thing to keep in mind though is that you can't provide development for external candidates to increase their qualifications and competencies for the job, but you do have that ability and that advantage for your internal employees as successor candidates. So this is another reason why succession planning primarily focuses on internal talent planning and development, to reduce the costs of external recruiting and to retain top talent.