Members are able to sign up on the front end of a NationBuilder website for a membership.
Members can also extend their membership on the front end, by paying for that membership before it expires.
It is currently not possible to import membership terms in order to extend a person membership by X amount of time, it remains a task that is only possible via the front end of the website.
This is a feature request for such a task be possible via the importer.
This is actually currently possible through the importer. In order to do this, you need to extract the membership ID of all of your members (this can be done by exporting your memberships) then update their 'expires_on' field to be the new expiration date. You must select the 'overwrite' checkbox so be very careful that every field you are mapping is meant to be replaced with the data in the file. It is also recommended to take a database snapshot before making a significant data change through the importer