Script that can be used in the browser console to add all the episodes on a Trakt season page to a list. (If used on a List page, it will remove all the episodes as well).

Trakt Add Episode to List.js

var lists = $('.trakt-icon-list-thick');
lists.each(function(){ 
  this.click(); 
  $(this).closest('a.list').siblings('.popover.with-list').find("li[data-list-id='{LIST_ID_HERE}']").click();
});

making public