public class POJO
{
  PRivate String field;
}

<introduction class="POJO">
   <interfaces>java.io.Serializable</interfaces>
</introduction>

 public class POJO
{
  private String field;
}

<introduction class="POJO">
   <mixin>
      <interfaces>
         java.io.Externalizable
      </interfaces>
      <class>ExternalizableMixin</class>
      <construction>new ExternalizableMixin(this)</construction>
   </mixin>
</introduction>

public class ExternalizableMixin implements java.io.Externalizable
{
   POJO pojo;

   public ExternalizableMixin(POJO pojo)
   {
      this.pojo = pojo;
   }

   public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException
   {
      pojo.stuff2 = in.readUTF();
   }
   public void writeExternal(ObjectOutput out) throws IOException
   {
      out.writeUTF(pojo.stuff2);
   }
}