Assuming you are still referring to the newsletter add-on then the following:

Admin>Customers>Subscription Manager then click on import.

Enter a sample record, using 'format' for email format field and 'email' for email.
Use NULL to indicate fields not to import.
Separate fields with a single space.
This needs to be applied to the following layout.

email NULL format NULL
A sample would then look like this:
(field 1)mail@somewhere~(field 2)blank~(field 3)HTML (or text)~blank
where ~ is space.