Переключитесь на процессор JSON JSON и сделайте следующее:
ObjectMapper mapper = new ObjectMapper();
// THIS IS WHAT I WAS LOOKING FOR TO HANDLE IN XSTREAM!!!!!!
mapper.configure(org.codehaus.jackson.map.DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);
User user = mapper.readValue(new File("user.json"), User.class);