You can import your custom fields per user through the CSV file used for the user import by following the below steps:
- First create some custom fields on the platform (Studio > Custom Fields)
- Grab the ID's of the custom fields you'd like to import. This is the first column in the table on the Custom Fields page.
- In the users' CSV file you can add the columns 'custom_field_id' and 'custom_field_value'
- For every user you can then assign the custom field value to the right custom field (defined by its ID)