Follow the steps for importing contacts with sub-accounts.

1. Export using the usual export button and open the Excel sheet.
2. filter the sheet according to Sub- Account TRUE/FALSE
3. First import sheet contains only FALSE sub accounts after DELETING duplicates using firstname, last name and email.
4. Second import sheet contains TRUE sub-accounts and Customer look up first name and last name are filled with the Parent account first name and last name ( from the export sheet).  DELETING duplicates using firstname, last name and email.

NOTE :  Make sure that parent account contains both first name and last name as the system will not map sub -accounts for parents without a last  name . So we need to ensure that from the exported excel sheet, we filter the parent account names only with first names and change it in the entire list.