Hi,
what you are trying to do is to add more information to mail that is sent as a task notification - am I correct?
If so, you could try to change the template that is used for notifications. You could use your own template and define it in the configuration files under the name task-notification.
I am not certain that it will work as an override of already defined mail template in default jBPM configuration but if not you could create your own jbpm.cfg.xml with custom defined notification template and build process engine using this file instead of default.
HTH
Maciej