如果文档已经有一些，如何添加新的CustomXmlDataStoragePropertiesPart？

using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Wordprocessing;
using System.IO;

public void AddCustomXmlData(string filePath)
{
    // 打开现有的Word文档
    using (WordprocessingDocument document = WordprocessingDocument.Open(filePath, true))
    {
        // 检查是否已经存在CustomXmlDataStoragePropertiesPart
        CustomXmlDataStoragePropertiesPart customXmlPart = document.MainDocumentPart.CustomXmlDataStoragePropertiesPart;

        if (customXmlPart == null)
        {
            // 如果不存在，则创建一个新的CustomXmlDataStoragePropertiesPart
            customXmlPart = document.MainDocumentPart.AddNewPart<CustomXmlDataStoragePropertiesPart>();
        }

        // 创建一个新的XML文档
        XmlDocument xmlDoc = new XmlDocument();
        XmlElement root = xmlDoc.CreateElement("CustomData");
        xmlDoc.AppendChild(root);

        // 添加一些自定义数据
        XmlElement item1 = xmlDoc.CreateElement("Item1");
        item1.InnerText = "Value1";
        root.AppendChild(item1);

        XmlElement item2 = xmlDoc.CreateElement("Item2");
        item2.InnerText = "Value2";
        root.AppendChild(item2);

        // 将XML文档保存到CustomXmlDataStoragePropertiesPart
        using (StringWriter stringWriter = new StringWriter())
        {
            xmlDoc.Save(stringWriter);
            customXmlPart.FeedData(new MemoryStream(Encoding.UTF8.GetBytes(stringWriter.ToString())));
        }
    }
}