您不能在初始化阶段之后添加其他servlet，因为servlet api规范（参见Javaservlet规范，版本4.0）禁止这样做。在第4.4节中 ServletContext#addServlet 方法是：
这些方法只能在应用程序初始化期间从 contexInitialized a方法 ServletContextListener 执行或从 onStartup a方法 ServletContainerInitializer 实施。
你仍然可以重复使用 ProxyServlet 类，但您需要编写自己的servlet（Map到 /* )，这将创建 ProxyServlet ，配置它们并将请求路由到正确的请求。
编辑：基本上你需要这样一个类：

public class MetaProxyServlet extends HttpServlet {

   /**
    * To configure a {@link ProxyServlet} without modifying its code.
    */
   private static class ProxyServletConfig implements ServletConfig {

      private static final String  P_TARGET_URI = "targetUri";
      private final String         targetUri;

      @Override
      public String getInitParameter(String name) {
         return P_TARGET_URI.equals(name) ? targetUri : null;
      }
      // Other methods
   }

   /**
    * Serves to modify the pathInfo of the request.
    */
   private static class RewrittenServletRequest extends HttpServletRequestWrapper {

      private final String pathInfo;

      @Override
      public String getPathInfo() {
         return pathInfo;
      }
      // Other methods
   }

   private TreeMap<String, ProxyServlet> proxyServlets = new TreeMap<>();

   /**
    * Adds a reverse proxy between {@code pathPrefix} of this servlets namespace
    * and {@code targetUri}.
    * 
    * @param pathPrefix
    *        must start with '/'
    * @param targetUri
    *        URI to be proxied
    * @throws ServletException
    */
   public void addRedirect(String pathPrefix, String targetUri) throws ServletException {
      final ProxyServlet servlet = new ProxyServlet();
      servlet.init(new ProxyServletConfig(pathPrefix, getServletContext(), targetUri));
      proxyServlets.put(pathPrefix, servlet);
   }

   @Override
   protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
      String pathInfo = req.getPathInfo();
      Entry<String, ProxyServlet> entry = proxyServlets.floorEntry(pathInfo);
      // entry.getKey() precedes, but not necessarily is a prefix for pathInfo
      if (entry != null && pathInfo.startsWith(entry.getKey())) {
         String rewrittenPathInfo = pathInfo.substring(entry.getKey().length());
         entry.getValue()//
              .service(new RewrittenServletRequest(req, rewrittenPathInfo), resp);
      } else {
         resp.sendError(HttpServletResponse.SC_NOT_FOUND);
      }
   }

}

完整的代码在要点上。