Add CDATA using XStream for specific fields

I am new to xStream and I have following code to transfer my custom object into String in xml format.

public static String textMessageToXml(ResponseTextMessage textMsg){
    xstream.alias("xml", textMsg.getClass());
    return xstream.toXML(textMsg);
}  

private static XStream xstream = new XStream(new XppDriver() {
        @Override
        public HierarchicalStreamWriter createWriter(Writer out) {  
            return new PrettyPrintWriter(out) {  
                // Add CDATA block  
                boolean cdata = true;//now I need to add cdata for some fields only  

                protected void writeText(QuickWriter writer, String text) {  
                    if (cdata) {  
                        writer.write("<![CDATA[");  
                        writer.write(text);  
                        writer.write("]]>");  
                    } else {  
                        writer.write(text);  
                    }  
                }  
            };  
        }  
    });

In ResponseTextMessage, I have several fields, the xml format I need is like

<xml>
<ToUserName><![CDATA[toUser]]></ToUserName>
<FromUserName><![CDATA[fromUser]]></FromUserName>
<CreateTime>12345678</CreateTime> <!--I do not need cdata for this field-->
<MsgType><![CDATA[text]]></MsgType>
<Content><![CDATA[content]]></Content>
</xml>

How can I achieve this using xStream? Do I have any way to control how to write xml in the format I hope based on the object's specific fileds?

Thank you!