Try this
<script type="text/javascript">
function AddItem()
{
// Create an Option object
var opt = document.createElement("option");
// Assign text and value to Option object
opt.text = "New Value";
opt.value = "New Value";
// Add an Option object to Drop Down List Box
document.getElementById('<%=DropDownList.ClientID%>').options.add(opt);
}
<script />
The Value will append to the drop down list.