A ver si sirve:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
<script>
function llenar(num){
document.getElementById('pp').options.length=0;
vec=['uno','dos','tres','cuatro','cinco','seis','siete','ocho','nueve','diez']
largo=(num==undefined)?vec.length:num
for(i=0;i<largo;i++){
document.getElementById('pp').options[i]=new Option(vec[i],vec[i])
}
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<select name="select" id="pp">
</select>
<input type="button" name="Submit" value="agregar" onclick="llenar()" />
<input type="button" name="Submit2" value="dejar 4" onclick="llenar(4)" />
</form>
</body>
</html>