Had trouble with too many temporary accounts on slack import, so hacked this together.

slack-import.js

var $selects = $('.user_map_selector select');
var throwAwayEmail = function(index) {
  return 'oldsupportacct+'+ getRandomInt(2500, 99999) +'@zencoder.com';
}

function getRandomInt(min, max) {
  return Math.floor(Math.random() * (max - min)) + min;
}

var count = 0;

$selects.each(function(index) {
  var $emailInput = $(this).closest('.user_map_selector').children("div").children("input");

  var currentVal = $(this).val();

  if (currentVal === '-') {
    count++;
    $(this).val('create');
    $emailInput.removeClass('hidden');
    $emailInput.val(throwAwayEmail(index));
  }
});

console.log(count);