Use the Item property to add custom attributes to your SiteMapNodes- here's a snippet from my custom provider:
' Create the SiteMapNode
itemNode = New SiteMapNode(Me, itemID, navigateUrl, itemName)
' Add additional info to sitemap node
itemNode.Item("itemType") = itemType
itemNode.Item("itemUrl") = itemUrl
Then you can access your attibutes in the same fashion:
Dim currNode As SiteMapNode = SiteMap.CurrentNode
reportType = currNode.Item("itemType")
reportUrl = currNode.Item("itemUrl")
Hope that helps.
Carl