Prefilling form with data

I am creating a XML form using the following tag and XSLT

<asp:Xml ID="XmlForm" runat="server" ></asp:Xml>

and in the code behind I get the XML and XSLT like this

' init the XML
  Dim xmlDoc As New XmlDocument
  xmlDoc.Load(Server.MapPath("mydata.xml"))
  'Used XPathNavigator to get the xmlDoc and
  'used the Transformsource to find the XSLT
   xmlForm.XPathNavigator = xmlDoc.CreateNavigator()
   XmlForm.TransformSource = Server.MapPath("/mytransform.xslt")

I was wondering how would I populate the fields if I am creating the form this way. Any idea or hints?