Need to add custom element in sitemap.xml file
If you don't mind a programmatic approach, then follow these steps:


Ensure that XML Sitemap module is installed &amp; enabled
Create/use a custom module (say, MYMODULE) &amp; use the following hook:

  function MYMODULE_xmlsitemap_element_alter(&amp;$element, $link, $sitemap) { 
    // Some logic to generate datetime
    // or hardcoded value
    $element['expires'] = '2010-10-10';
  }

Go to admin/config/search/xmlsitemap/batch-generate &amp; click on 'Run'
Go to admin/config/search/xmlsitemap/rebuild &amp; click on 'Rebuild sitemap'
Go to sitemap.xml, inspect element or view page source &amp; verify that 'expires' tag is included


Hope this helps.