Create some apex logic for old Accounts, so we can run this logic,get all accounts without 'custom users" then create these 'custom users', link them with the correspondent account and link all opportunities from Account record to the custom user.

 Create inner class with comparator (by lastName asc) for parsing this json:

[{"lastName" : "Perucci", "firstName" : "John", "title" : "guitarist" },{"lastName" : "LaBrie", "firstName" : "James", "title" : "singer"},{"lastName" : "Mabgini", "firstName" : "Mike", "title" : "drummer"},{"lastName" : "Myung", "firstName" : "John", "title" : "basist"}]

Create some apex logic for old Accounts, so we can run this logic,get all accounts without 'custom users" then create these 'custom users', link them with the correspondent account and link all opportunities from Account record to the custom user.