Solution : https://service.sap.com/sap/support/notes/1012708 (SAP Service marketplace login required)
Key words : 
external person id reason, attached correction instructions, transaction pa40, user locks, user opens, terms pa40, rp50g-personid_ext, prerequisites prerequisite, solution import, support package
Related Notes : 
       
| 1261193 | |
| 1245560 | Composite SAP Note : XSS Documentation |