XmlDataProvider provider = new XmlDataProvider(xmlDocument);
provider.PreParseXPathNavigator += new EventHandler<PreParseXPathNavigatorEventArgs>(provider_PreParseXPathNavigator);
...
void provider_PreParseXPathNavigator(object sender, PreParseXPathNavigatorEventArgs e)
{
    if (e.Navigator.Name == "ZIP")
    {
        e.ParsedType = typeof(string);
        e.ParsedContent = e.Navigator.ValueAs(typeof(string));
        e.SkipDefaultProcessing = true;
    }
}