To import Customers from the Admin / Manage Import Routines and Admin / Import Data menu options use the Account Type field in the routine and set this to 1 in the import file for Individuals and 0 for Organisations.
Customers set as account type Organisation have the name stored only in the NAME field of the Customers table.
Customers set as account type Individual have the name stored in the fields GIVENNAME (for the first name) and SURNAME (for the last name) and then get these values concatenated and stored in the NAME field as well.
This means that for Organisation accounts, the fields GIVENNAME and SURNAME are empty. If an import is made to change the account type from Organisation to Individual, the import should also contain columns to bring in the GIVENNAME and SURNAME.