How do I put a maiden name in a person's contact card, and what is the field name for the maiden name in the NPSP Date Import Template?

There's no standard field for Maiden Name or Nickname in Salesforce, but it's very easy to add custom fields to the Contact Object

See https://trailhead.salesforce.com/en/content/learn/projects/suggestion_box/suggestion_box_2 for a step by step guide