How to send custom XML data using the Smack library?

I'm using the Smack API in Java to connect to my XMPP server.

I want to send a customized message packet like this:

<message to="you@MyServer.com" type="chat" MYFIELD="custom stuff">
    <body> hi </body>
    <CUSTOM_STANZA A="..." B="..."> 
        C="..." 
        D="..."
    </CUSTOM_STANZA>
</message>

I'm guessing that I create implement my own Packet with that returns this XML in it's toXML() method. But that doesn't seem to work.

Any help would be appreciated.