There are two ways you can keep track of data that doesn't correspond to any of the fields available for import: tags or custom fields.

Tags: Tags are best used for when you have information that can be applied to multiple people in your database. For example, I might have a tag like, "Attended kickoff event" or, "Blue Team" because the same terms apply to a group of people in my database.

Tags will also be easily viewable in your People section since they'll be listed directly next to a profile's name:

Additionally, tags can be applied when your users take actions on your website (if you choose to apply a tag to people who interact with a page), or added and removed dynamically through saved filters.

If you're on the Team level plan or higher, you can also use tags to share your data to other nations.

Custom fields: Custom fields are more useful when you have data that is unique to individual profiles or to a very limited number of profiles.

For example, I might have a custom field for "Pet's name" or "Number of children." Even though multiple people in my database might have 2 kids, this likely isn't information I need to see immediately when I'm interacting with my supporters. 

Custom fields can be filtered for and imported into. The data is stored under Edit > Custom fields in individual profiles:

To import data to a custom field, you will first need to create the custom field in your database. It will then become available as a mapping option.

To import tags, you can map the column to tag_list with each tag separated by comma. If you have a tag (or tags) that you would like to apply uniformly to every profile in the import, you can enter them (again, separated by commas) to the "Add tags to each person" field on the mapping screen of your import.