Add Custom Field to Title

Hi,
Is it possible to add a custom field to the page title in the sitemap?
For instance, one site has a custom field called the Subtitle.
I tried using this code, but it is not working:
<a href="{permalink}">{title}: {cf_subtitle}</a>
Please let me know what you recommend.
Thanks!

http://wordpress.org/plugins/wp-sitemap-page/