<script language="javascript" type="text/javascript">
function itemsadd()
{     
var myForm = document.Form1; //Form Name
var mylist = myForm.listbox1;// Listbox Name    
var myOpt;            
myOpt = document.createElement("Option");                
if(document.getElementById("ddl1").selectedIndex > 0)
{
myOpt.text = document.getElementById("ddl1").value;  //text of dropdownlist
myOpt.value = document.getElementById("ddl1").selectedIndex;  //index value of dropdownlist
mylist.add(myOption); add item of dropdown list into listbox
}
</script>

On Default.aspx.cs Page:

page_load()
{
ddl1.attributes.add("OnChange","itemsadd");
}