How to add additional fields to form before submit?

Try this:

$('#form').submit(function(eventObj) {
    $(this).append('<input type="hidden" name="field_name" value="value" /> ');
    return true;
});

You can add a hidden input with whatever value you need to send:

$('#form').submit(function(eventObj) {
    $(this).append('<input type="hidden" name="someName" value="someValue">');
    return true;
});

Yes.You can try with some hidden params.

  $("#form").submit( function(eventObj) {
      $("<input />").attr("type", "hidden")
          .attr("name", "something")
          .attr("value", "something")
          .appendTo("#form");
      return true;
  });

$('#form').append('<input type="text" value="'+yourValue+'" />');