add title attribute from css

Well, although it's not actually possible to change the title attribute, it is possible to show a tooltip completely from CSS. You can check a working version out at http://jsfiddle.net/HzH3Z/5/.

What you can do is style the label:after selector and give it display:none, and set its content from CSS. You can then change the display attribute to display:block on label:hover:after, and it will show. Like this:

label::after {
  content: "my tooltip";
  padding: 2px;
  display: none;
  position: relative;
  top: -20px;
  right: -30px;
  width: 150px;
  text-align: center;
  background-color: #fef4c5;
  border: 1px solid #d4b943;
  -moz-border-radius: 2px;
  -webkit-border-radius: 2px;
  -ms-border-radius: 2px;
  border-radius: 2px;
}
label:hover::after {
  display: block;
}

Quentin is correct, it can't be done with CSS. If you want to add a title attribute, you can do it with JavaScript. Here's an example using jQuery:

$('label').attr('title','mandatory');

You can't. CSS is a presentation language. It isn't designed to add content (except for the very trivial with :before and :after).