Click on Individuals followed by the Settings icon

Once within the Individual Settings you can add more by selecting + additional field.

Add a field name then the type of field you would like to use. Once selected click the Create button.

You will now be returned to the additional fields page and will see the new field within the list.

If you now head to the Individuals tab and select a user, you can enter the dropdown menu and select the Additional data tab to view the additional data which is against an individual, whilst in here you can also set and update this data.