Most online generators just run a spider and follow the links. You would need to find a generator built into the CMS that looks at the database for all the pages the site has. You could also do it manually by looking at the tables and building the URLs. If you want a simple sitemap that does not have priority or frequency then have a look at site map builder . net. It’s a .net application that runs locally and totally free.
You can also read How do we create Google Sitemap in Magento?
You can also use Extended Sitemap Magento Extension – The one and comprehensive solution for Magento!
Extended Sitemap allows you to display all of your existing Catalog Categories, Products, CMS Pages and additional configurable links with a hierarchical structure SEO optimized for web spiders on a single Site Map page available through your store footer link, e.g. http://www.example.com/sitemap/
In addition it replaces the default Magento Google Sitemap functionality for auto generation of XML Site Map files with a more optimized and advanced functionality which eases user experience.
http://www.magentocommerce.com/extension/1977/extended-sitemap
