How to avoid xmlns="" in an XML element added?

I have 287 SSRS reports (XML files), where I need to add a parameter.
They all start like this:

<?xml version="1.0" encoding="utf-8"?>
<Report xmlns="http://schemas.microsoft.com/sqlserver/reporting/2005/01/reportdefinition" xmlns:rd="http://schemas.microsoft.com/SQLServer/reporting/reportdesigner">
<DataSources>
<DataSource Name="COR_Basic">
<rd:DataSourceID>addde073-f37c-4b59-ae3a-25231ffc0ec6</rd:DataSourceID>
<DataSourceReference>COR_Basic</DataSourceReference>
</DataSource>
</DataSources>
<InteractiveHeight>29.7cm</InteractiveHeight>
<ReportParameters>
<ReportParameter Name="in_report_name">
<DataType>String</DataType>
<DefaultValue>
<Values>
<Value>=Globals!ReportName</Value>
</Values>
</DefaultValue>
<Prompt>Report Name</Prompt>
<Hidden>true</Hidden>
</ReportParameter>
<ReportParameter Name="in_mandant">
<DataType>String</DataType>
<DefaultValue>
<Values>
<Value>0</Value>
</Values>
</DefaultValue>
<Prompt>in_mandant</Prompt>
<Hidden>true</Hidden>
</ReportParameter>

Now I want to automagically add the "proc" parameter to all reports.
Which means I need to insert this bit of XML

<ReportParameter Name="proc">
<DataType>String</DataType>
<DefaultValue>
<Values>
<Value>MyTestValue</Value>
</Values>
</DefaultValue>
<Prompt>MyPromptText</Prompt>
<Hidden>true</Hidden>
</ReportParameter>

after

<ReportParameter Name="in_mandant">
...
</ReportParameter>

This is the code I have so far:

public static System.Xml.XmlDocument File2XmlDocument(string strFileName)
{
    // http://blogs.msdn.com/b/tolong/archive/2007/11/15/read-write-xml-in-memory-stream.aspx
    System.Xml.XmlDocument doc = new System.Xml.XmlDocument();
    // doc.Load(memorystream);
    // doc.Load(FILE_NAME);

    using (System.Xml.XmlTextReader xtrReader = new System.Xml.XmlTextReader(strFileName))
    {

        doc.Load(xtrReader);
        xtrReader.Close();
    } // End Using xtrReader

    return doc;
} // End Function File2XmlDocument




        public static System.Xml.XmlNamespaceManager GetReportNamespaceManager(System.Xml.XmlDocument doc)
        {
            System.Xml.XmlNamespaceManager nsmgr = new System.Xml.XmlNamespaceManager(doc.NameTable);
            nsmgr.AddNamespace("dft", "http://schemas.microsoft.com/sqlserver/reporting/2005/01/reportdefinition");

            return nsmgr;
        } // End Function GetReportNamespaceManager



 public static void AddProc(string strFilename)
        {
            System.Xml.XmlDocument doc = File2XmlDocument(strFilename);
            System.Xml.XmlElement root = doc.DocumentElement;
            System.Xml.XmlNamespaceManager nsmgr = GetReportNamespaceManager(doc);

            System.Xml.XmlNode xnMandant = root.SelectSingleNode("/dft:Report/dft:ReportParameters/dft:ReportParameter[@Name=\"in_mandant\"]", nsmgr);

            string strReportName = System.IO.Path.GetFileNameWithoutExtension(strFilename);

            if (xnMandant != null)
            {
                LogMessage("{0}\t{1}", strReportName, xnMandant.FirstChild.Value);

                string frag = @"<ReportParameter Name=""proc"">
<DataType>String</DataType>
<DefaultValue>
<Values>
<Value>MyTestValue</Value>
</Values>
</DefaultValue>
<Prompt>MyPromptText</Prompt>
<Hidden>true</Hidden>
</ReportParameter>";

                System.Xml.XmlDocumentFragment xmlDocFrag = doc.CreateDocumentFragment();
                xmlDocFrag.InnerXml = frag;

                System.Xml.XmlNode xn = xnMandant.ParentNode.InsertAfter(xmlDocFrag, xnMandant);

                Console.WriteLine(xn.OuterXml);

            }
            else
                LogMessage("{0}\tKein Parameter in_mandant", strReportName);


            SaveDocument(doc, strFilename);
        } // End Sub PrintStichtag

This inserts the XML-Fragment at the right location, but I have a xmlns="" after "proc".

<ReportParameter Name="proc" xmlns="">
<DataType>String</DataType>
<DefaultValue>
<Values>
<Value>MyTestValue</Value>
</Values>
</DefaultValue>
<Prompt>MyPromptText</Prompt>
<Hidden>true</Hidden>
</ReportParameter>

How can I avoid the empty xmlns attribute ?
(Short of loading the xml file as text and doing string.replace) .