Instead of specifying mails in the processdefinition.xml, mails can be specified in a template file. When a template is used, each of the fields can still be overwritten in the processdefinition.xml. The mail templates should be specified in an XML file like this:

<mail-templates>

  <variable name="BaseTaskListURL" value="http://localhost:8080/jbpm/task?id=" />

  <mail-template name='task-assign'>
    <actors>#{taskInstance.actorId}</actors>
    <subject>Task '#{taskInstance.name}'</subject>
    <text><![CDATA[Hi,
Task '#{taskInstance.name}' has been assigned to you.
Go for it: #{BaseTaskListURL}#{taskInstance.id}
Thanks.
---powered by JBoss jBPM---]]></text>
  </mail-template>

  <mail-template name='task-reminder'>
    <actors>#{taskInstance.actorId}</actors>
    <subject>Task '#{taskInstance.name}' !</subject>
    <text><![CDATA[Hey,
Don't forget about #{BaseTaskListURL}#{taskInstance.id} 
Get going !
---powered by JBoss jBPM---]]></text>
  </mail-template>

</mail-templates>    
    

As you can see in this example (BaseTaskListURL), extra variables can be defined in the mail templates that will be availble in the expressions.

The resource that contains the templates should be configured in the jbpm.cfg.xml like this:

<jbpm-configuration>
  ...
  <string name="resource.mail.templates" value="jbpm.mail.templates.xml" />
</jbpm-configuration>