There is no way to add querystring parameters to the SiteMapNode in a SiteMap control "out of the box." I'm quite surprised there is no option but luckily the SiteMap uses the provider model. After a Live search, I was able to find a solution
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Read the complete post at http://feeds.feedburner.com/~r/JasonNGaylord/~3/305023890/adding-querystring-parameters-to-the-sitemapnode.aspx