Is it an obligation using Data Import Tool? You can use Data Management Tool instead, it's something that is designed to create new entries (not modify though).
DataManagement Tool is to be installed on your desktop, please check this thread for details:
Some Excel files are designed for the cataloges (foundation folder):
ProductCatalog.xls, OperationalCatalog.xls, GenericCatalog.xls.
Then use the "Data Load Console" to convert the excel file to csv, import it to ARS "staging" forms and validate data and promote them.
The Data Load Console is on ARS, log in with an administrator account.
Application AdministrationConsole -> Custom Configuration -> Foundation -> Data Management Tool -> Data Load Console
If you really want to use Data Import Tool, you can find the form names in the Excel files but well... You'll be missing all the checks, verifications that Data Management Tool and Data Load Console offer you.