how do i create and insert an xml node inside another xml node c# w/xmldocument?

I'm trying to insert an xml node with another child xml node in an existing xmldocument in C#. I've got an XML doc that looks like so:

<?xml version="1.0" encoding="utf-16"?>
<DictionarySerializer>
    <item>
        <key>statusCode</key>
        <value>0</value>
    </item>
    <item>
        <key>statusSeverity</key>
        <value>INFO</value>
    </item>
    <item>
        <key>statusMessage</key>
        <value>Status OK</value>
    </item>
    <item>
        <key>MerchantAccountNumber</key>
        <value>9999999999</value>
    </item>
    <item>
        <key>ReconBatchID</key>
        <value>420150418 1Q02144266965047801046AUTO04</value>
    </item>
    <item>
        <key>PaymentGroupingCode</key>
        <value>3</value>
    </item>
    <item>
        <key>responsePaymentStatus</key>
        <value>Completed</value>
    </item>
    <item>
        <key>TxnAuthorizationTime</key>
        <value>2015-04-18T09:14:41</value>
    </item>
    <item>
        <key>TxnAuthorizationStamp</key>
        <value>1429348481</value>
    </item>
    <item>
        <key>ClientTransID</key>
        <value>aidjl79f</value>
    </item>
</DictionarySerializer>

and I need to insert another node with a node and node at the bottom. I have this so far:

  XmlDocument xmlCustomValues = new XmlDocument();
            xmlCustomValues.LoadXml(OldCustomValues);
            XmlNode NodeItem = xmlCustomValues.SelectSingleNode("DictionarySerializer");
            XmlNode NodeNewItem = xmlCustomValues.CreateNode(XmlNodeType.Element, "item", null);
    XmlNode NodeNewKey = NodeNewItem.??????

but not sure how to create a node under NodeNewItem (there's no "CreateNode" method). Never done this before (obviously) and the syntax does not make sense to me.