FOrgive me if I am not understanding your question, but can't you just add a parent element " to contain all strings?
Dim sbUser As New System.Text.StringBuilder
sbUser.AppendLine("<?xml version=""1.0""?>")
sbUser.AppendLine("<root>")
sbUser.AppendLine("<services>")
For Each item As KeyValuePair(Of Integer, Users) In dictusers
sbUser.AppendFormat("<Name>""{0}""<SessionID>""{1}""</SessionID><UserName>{2}</UserName></Name>", item.Key, item.Value.SessionID, item.Value.UserName).AppendLine()
Next
sbUser.AppendLine("</services>")
sbUser.AppendLine("<cpus>")
...
sbUser.AppendLine("</cpus>")
...
sbUser.AppendLine("</root>")