In Livecycle ES, the product only provides standard e-mail message templates that can be configured using the LiveCycle Admin Console
for task assignment and reminders.
In LiveCycle ES Update 1, we have provided the ability to configure these messages on a task/process basis.
I presume you are still running LiveCycle ES and hence you will need to upgrade to LiveCycle ES Update 1 for this functionality. There are work-arounds in LiveCycle ES leveraging task assignment events however this would require some customization.
Product Management, LiveCycle ES