iTextSharp - Retain PDF Metadata (PDF Creator, Last Modified, ...)

Question

I am writing a little archiver for PDF documents and would like to add a few Keywords to the metadata.

I used code from Read/Modify PDF Metadata using iTextSharp but when adding them, some metadata is overridden (ie ModDate). Is there a way to suppress this behaviour? This is not helpful for (ie searching/ filtering) documents lateron.

PDF Producer: iText 1.4 (by lowagie.com); modified using iTextSharp (TM) 5.5.6 (c) 2000-2014 iTextGroup NV (AGPL-version)

Code I use to add keywords

    private void Button_Click_ADD_KEYWORDS(object sender, RoutedEventArgs e)
    {
        using (MemoryStream m = new MemoryStream())
        {
            using (PdfReader pdfReader = new PdfReader(@"c:\sim\input.pdf"))
            using (PdfStamper pdfStamper = new PdfStamper(pdfReader, m))
            {
                pdfStamper.Writer.CloseStream = false;

                Dictionary<String, String> info_orig = pdfStamper.Reader.Info;
                Dictionary<String, String> info_new = new Dictionary<String, String>();

                foreach (String key in info_orig.Keys)
                {
                    if(key.ToLower().Equals("keywords"))
                    {
                        info_new["Keywords"] = info_orig["Keywords"] + "; " + this.AddKeywords();
                    }
                    else
                    {
                        info_new[key] = info_orig[key];
                    }
                }

                info_orig.Clear();

                pdfStamper.MoreInfo = info_new;
            }

            m.Seek(0L, SeekOrigin.Begin);

            using (FileStream f = new FileStream(@"C:\sim\output.pdf", FileMode.Create))
            {
                m.CopyTo(f);
            }
        }

        MessageBox.Show("I think it worked ...");
    }