At the moment, it's not possible to create custom fields via an import - so the custom fields need to already exist in Pipedrive.
If they do already exist, then you can import new Leads into your account and simply map that custom field to the wanted field when importing.
If you are looking to update information on your Leads, in that case, the best option would be to Export all your leads via the Export tool (as it will contain an ID) and add another column into your CSV file for the custom field and re-import the information - all the previous leads will then have new information added.
NOTE: It's not possible to export from the Lead view, the Pipedrive ID System, so it must be done via the Export Tool.
And of course, if further assistance is needed, our Support team is available 24/7 via our in-app chat system or via our email [email protected]
All the best,