Can managers add Successors that are outside of their heirarchy?

Summary

Can managers add Successors that are outside of their heirarchy?

Content

It appears that managers adding successors to a succession plan for their direct report can only add Internal Successors that are part of their hierarchy? If they want to add someone from a different organization they cannot do so. They also have External Caniddates as an option but external candidates have to be added by HR Specialist. Is there a better process then this? For a manager, they are limited on candidates they can add to a succession plan without having to reach out to HR Specialist to add someone as a External Candidate.