怎么使用SharpZipLib.Silverlight4向目标压缩包追加文件？

目前遇到一个需求要求将文件追加到压缩包中去，使用的是SharpZip组件，ZipOutputStream流存储，我做了一个测试，代码如下：
private void button1_Click(object sender, RoutedEventArgs e)
        {
            SaveToIs();
        }

        private void btnAddFile_Click(object sender, RoutedEventArgs e)
        {
            AddToZip();
        }

        /// <summary>
        /// 文件名
        /// </summary>
        public static string Name { get; set; }

        /// <summary>
        /// 保存到存储器
        /// </summary>
        public static void SaveToIs()
        {
            Stream contentStream = null;
            OpenFileDialog sfd = new OpenFileDialog
            {
                Filter = "文件|*.*",
            };
            if (sfd.ShowDialog() == true)
            {
                Name = sfd.File.Name;
                contentStream = sfd.File.OpenRead();
                SetStream(contentStream, Name);
            }
        }

        /// <summary>
        /// 从存储器读取
        /// </summary>
        /// <param name="name"></param>
        /// <returns></returns>
        public static Stream GetStream(string name)
        {
            Stream stream;
            using (IsolatedStorageFile myIsf = IsolatedStorageFile.GetUserStoreForApplication())
            {
                //检查是否存在
                if (!myIsf.FileExists(name))
                {
                    return null;
                }
                //打开流
                var myIsfs = new IsolatedStorageFileStream(name, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete, myIsf);

                stream = myIsfs;
            }
            return stream;
        }

        /// <summary>
        /// 保存到存储器
        /// </summary>
        /// <param name="stream"></param>
        /// <param name="name"></param>
        public static void SetStream(Stream stream, string name)
        {
            Stream fileStream = stream;
            using (var myIsf = IsolatedStorageFile.GetUserStoreForApplication())
            {
                using (var myIsfs = new IsolatedStorageFileStream(name, FileMode.OpenOrCreate, myIsf))
                {
                    fileStream.CopyTo(myIsfs);//拷贝到独立存储
                    fileStream.Close();
                }
            }
        }

        /// <summary>
        /// 添加至zip
        /// </summary>
        public static void AddToZip()
        {
            OpenFileDialog sfd = new OpenFileDialog
            {
                Filter = "压缩包|*.zip"
            };
            if (sfd.ShowDialog() == true)
            {
                Stream contentStream = new FileInfo(sfd.File.FullName).Open(FileMode.OpenOrCreate, FileAccess.ReadWrite);
                var fs = GetStream(Name);
                var s = new ZipOutputStream(contentStream);
                var buffer = new byte[fs.Length];
                fs.Read(buffer, 0, buffer.Length);
                var entry = new ZipEntry(Name);
                s.SetLevel(9);
                entry.CompressionMethod = CompressionMethod.Stored;
                entry.IsUnicodeText = true;
                fs.Close();
                s.PutNextEntry(entry);
                s.Write(buffer, 0, buffer.Length);
                fs.Close();
                s.Close();
            }
        }
但是这种做法很不理想，使用了
s.PutNextEntry(entry);
s.Write(buffer, 0, buffer.Length);
实现的是覆盖之前zip中的流的buffer.Length的大小。
由此想到在获取filestream，并且在创建ZipOutputStream之前，将filestream的Position指定到末尾位置，是不是就能实现追加，但是这种做法调试之后，压缩包损坏了。
有没有兄弟使用过SharpZip追加的援助一下！~ 压缩 silverlight ZipFile SharpZip

补充：.NET技术 ,  C#