Add XML Node using another XML file

I have 3 xml files which is stored in one location. I want to add all these 3 XML into one XML file with different node name. I could get the XML values as string using XMLTextWriter and StringWriter; but adding to the new xml as node with this string is causing problems. The following :

 Private Sub btnXMLtoStr_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnXMLtoStr.Click

        Dim strXML As String

        Dim oXML As New XmlDocument()
        Dim oRoot As XmlNode
        Dim oNode As XmlNode
        Dim oDB As XmlNode
        Dim oTable As XmlNode

        oRoot = oXML.CreateElement("WindcatcherSettings")
        oNode = oXML.AppendChild(oRoot)

        oDB = oXML.CreateElement("ThresholdXML")
        strXML = GetXMLString("C:\Thresholds.xml")

        oNode.AppendChild(oDB)

        oTable = oXML.CreateElement(strXML)
        oDB.AppendChild(oTable)

        oXML.Save("C:\AllSettings.xml")
    End Sub


    Private Function GetXMLString(ByVal XMLPath As String) As String

        Dim xmlDoc As New XmlDocument()
        Dim sw As New StringWriter()
        Dim sVal As String


        Try
            xmlDoc.Load(XMLPath)
            Dim xw As New XmlTextWriter(sw)
            xmlDoc.WriteTo(xw)
            sVal = sw.ToString()
        Catch ex As Exception

        End Try

        GetXMLString = sVal
    End Function

Error is thrown on link         oTable = oXML.CreateElement(strXML)

After writing is it possible to retrieve all the node values as string? Say I want to search for "ThresholdXML" and get all the node values as String? Please help