<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
<sitecore>
    <customHandlers>
        <handler trigger="sitemap.xml" handler="sitemap.ashx" />
    </customHandlers>
</sitecore>
</configuration>

<add path="sitemap.ashx" verb="*" type="MySite.CustomSitecore.Handlers.SitemapHandler,MySite" name="SitemapXml" />

namespace MySite.CustomSitecore.Handlers
{
public class SitemapHandler : IHttpHandler
{
    public bool IsReusable
    {
        get
        {
            throw new NotImplementedException();
        }
    }

    public void ProcessRequest(HttpContext context)
    {
        try
        {
            var curSite = Sitecore.Context.Site;
            var m_Sites = SitemapManagerConfiguration.GetSites();
            foreach (DictionaryEntry site in m_Sites)
            {
                if (site.Key.ToString().Equals(curSite.Name))
                {
                    var filepath = site.Value;
                    HttpContext.Current.Response.Redirect("/" + filepath, false);
                    return;
                }
            }
            return;
        }
        catch (Exception ex)
        {
            Log.Info("Error in SitemapHandler: " + ex, this);
        }
    }
}
}

HttpContext.Current.Response.StatusCode = 200;
HttpContext.Current.Response.ContentType = "text/xml";
HttpContext.Current.Response.AddHeader("Location", "/" + filepath);
HttpContext.Current.Response.End();
return;