Yes, the sitemap format is its own format - you cannot use the atom feed for that. You need to install the google sitemap plugin for that one to be created, I'm not sure if its available on supersized?
Supersized doesn't have FTP/filesystem access. Did you check the plugin listing for serendipity_event_google_sitemap? If it's not there, you might need to contact the supersized-admin via mail (http://supersized.org/disclaimer.php).