Try something like this:
var typeElem = responseElem[0].getElementsByTagName("Table");
for (var i = 0; i <typeElem.length;i++)
{
var textNode = document.createTextNode(typeElem[i].firstChild.text);
AppendToSelect(typeElem[i].lastChild.text, textNode);
}
function AppendToSelect(value, content)
{
var opt;
customerList = window.document.getElementById("ListBox");
opt = document.createElement("option");
opt.value = value;
opt.appendChild(content);
customerList.appendChild(opt);
}
You should be able to get the values selected during postback.