Merging External candidate to Internal current employee

Summary

Internal candidate applied as external, how do we merge the two

Content

We had an internal employee who applied as external, and the external is post offer ready to to be moved to HR, is there a way to merge the external candidate to internal? i'm not too sure what is my option here. i can manually create a "promote" record in HR, but i'd like to finish the external and merge it with internal somehow. 

Version

20C