I have the admin certification, going for app builder hopefully by end of the month.
Don't believe you can get an auto-number for an import but saw this if it might help you
Set the field up as a text field first, import your data, then change the data type of the field to auto number. Start the auto number sequence at the next number you want and be sure to not backfill existing records. Your pre-existing values will stay in tact after converting the field type.