Use the options –

Copy/paste from file – copy and paste contacts from .xls and .xlsx
– copy from the excel sheet and paste into mailchimp. Later assign the column names.
Additional columns can be added later. For eg. Date of mailchimp joining etc.
– user email and the additional column to update. choose update contact if existing in list.
– when importing date use the format YYYY-MM-DD else the date will not get imported