You can use this code to add a value into your .csv file upon export.

add_filter('frm_csv_value', 'frm_csv_value', 10, 2);
function frm_csv_value($value, $atts){
  if($atts['field']->id == 25){ //change 25 to your field id
    $value = 'new value here';
  }
return $value;
}