The same way you would subscribe to any mbean. The emitted notification is of type jboss.alarm.table.update. You could extend the ListenerServiceMBeanSupport baseclass (look in the wiki).
The JMXNotificationAppender is itself an MBean, to which the AlarmTable subscribes to get the notifications containing the log messages.
If you add a second appender you need to configure the AlarmTable subscription list accordingly.