The import does not support extra fields, as per examples in the import panel.
There is not a feature to import your specific data with extra fields directly available in the plugin admin side.
To import your specific data, you should work at the database level or check if there is an extension in the WordPress repo made by a third party.
Do we lack of that feature? Yes we do. That is why there is no information on how to import extra fields. Will that more powerful import available in the future? Could be, we have no a scheduled date for that.
Why there is not an import for extra fields? The extra field number can be change and the input format cannot be standardized. That implies to make a configurable importer which pre-reads the file and lets to map CSV columns to Newsletter subscriber fields.
Last note: if you add a post in the forum we receive a notification, as explained, and we can miss it.
If you add replies to your self, we do not receive notifications. It’s more useful to open another topic referring to the unanswered one: we will be notified and unlikely we can miss two requests and then we can merge the forum topic and answer.
If we insist ignoring you (as you said), but we do not ignore people, open a support ticket in the WordPress support forum available to every plugin published in their repo:
https://www.ads-software.com/support/plugin/newsletter
Again we receive a notification for each request.
As always, all that for free.
Have a nice day, Stefano.