So finally I found a Solution by myself, you can use AltChunk to Insert Plaintext and not losing your formatting :-)
Here is the Code maybe it helps someone out who has the same Problem ;-)
public static void SetNotes(this WordprocessingDocument doc, string value)
{
MainDocumentPart main = doc.MainDocumentPart;
string altChunkId = "AltChunkId" + Guid.NewGuid().ToString().Replace("-", "");
var chunk = main.AddAlternativeFormatImportPart(AlternativeFormatImportPartType.TextPlain, altChunkId);
using (var mStream = new MemoryStream())
{
using (var writer = new StreamWriter(mStream))
{
writer.Write(value);
writer.Flush();
mStream.Position = 0;
chunk.FeedData(mStream);
}
}
var altChunk = new AltChunk();
altChunk.Id = altChunkId;
OpenXmlElement afterThat = null;
foreach (var para in main.Document.Body.Descendants<Paragraph>())
{
if (para.InnerText.Equals("Notes:"))
{
afterThat = para;
}
}
main.Document.Body.InsertAfter(altChunk, afterThat);
}