In webwork.properties(as of Webwork 2.1.1),
you can now use:

webwork.custom.i18n.resources=global-messages

Serveral resource bundles can be specified by comma separating them.
for example see webwork.properties :
http://wiki.opensymphony.com/display/WW/webwork.properties

Java class (thanks Drew McAuliffe):

public class WebworkGlobalMessagesListener implements ServletContextListener {
    private static Logger log = Logger.getLogger(WebworkGlobalMessagesListener.class);
    private static final String DEFAULT_RESOURCE = "global-messages";

    /**
     * Uses the LocalizedTextUtil to load messages from the global
     message bundle.
     * @see
     javax.servlet.ServletContextListener#contextInitialized(javax.servlet.Servle
     tContextEvent)
     */
    public void contextInitialized(ServletContextEvent arg0) {
        log.info("Loading global messages from " + DEFAULT_RESOURCE);
        LocalizedTextUtil.addDefaultResourceBundle(DEFAULT_RESOURCE);
        log.info("Global messages loaded.");
    }

    /**
     * @see javax.servlet.ServletContextListener#contextDestroyed(javax.servlet.ServletContextEvent)
     */
    public void contextDestroyed(ServletContextEvent arg0) {

        // do nothing
    }

}

web.xml:
(under listeners section)

<listener>
<listener-class>mypackagename.WebworkGlobalMessagesListener</listener-class>
</listener>