I'm not sure if the sitemap itself is extensible. There are options to create providers for the sitemap, to get URLs into the sitemap from custom URLs, but I don't know that you can pass additional info over to it for other attributes.

public override List<SitemapUrl> GetUrls(int portalId, DotNetNuke.Entities.Portals.PortalSettings ps, string version)

        {

            var listOfUrls = new List<SitemapUrl>();

            foreach (var a in Article.GetAllArticlesList(portalId))

            {

                var pageUrl = new SitemapUrl

                                  {

                                      Url = Utility.GetItemLinkUrl(a.ItemId, portalId),

                                      Priority = (float)0.5,

                                      LastModified = Convert.ToDateTime(a.LastUpdated),

                                      ChangeFrequency = SitemapChangeFrequency.Daily

                                  };

                listOfUrls.Add(pageUrl);

            }

            return listOfUrls;

        }