CRM4.0 mail merge for system entities like incident, product

I was trying to write a mail merge tempelate for incident and noticed that incident does not appear in the list for Associated Entity while creating a template. For all custom entities, mail merge is enable by default but for system entities. If you don’t see the entity name in the dropdown, follow these steps:

•Export out the customizations for the entity for which you want to write the mail merge template.
•Open the zip file and extract the customization.xml present inside it.
•Update the “IsMailMergeEnabled” node value from 0 to 1 i.e. 1
•Import the updated customization.xml file back into CRM via import customizations.
•Publish the entity after import.
•The mail merge support for that entity will be switched on now and entity name will appear in the drop down while creating template.