E' più una domanda da javascript, comunque:

<html>
<head>

<script>
function add(){
a = document.getElementById("pippo");
b = document.createElement("option");
a.appendChild(b);
b.text = "four";
b.value = 4;
alert(b.value);
}
</script>
</head>
<body>

<form name="fm">

<select id="pippo">
<option value="1">uno</option>
<option value="2">due</option>
<option value="3">tre</option>
</select>


<input type="button" value="prova" onClick="add();">

</form>

</body>