I had asked this question to out Solutions Architect -- the answer as of a few weeks ago was this is potentially in the works, but for now the only way to modify the email template is via executing a script. You'll need to create a script to perform what you want and keep it on the ESM backend and for your actions on the alarm, have it run the script. I've yet to set one up, but when I do I'll post my findings.
There is another way to achieve customized content in email from an alarm today, by leveraging a custom report. "Generate Report" is an often-overlooked alarm action that can be very helpful, depending on your use case. If you can create a customized report with the content you need, you can then set up your alarm to automatically generate the report at the time the alarm is triggered, and email it to the desired recipients. This gives you pretty complete control over the content that is included in your email, although obvioulsy it comes as an attachment rather than in the body of the email itself.
A minor correction regarding scripts as an alarm action: alarm action scripts are not typically run on the ESM; the scripts are more often run on a bastion host. When an alarm with a script action is triggered, the ESM authenticates to the bastion host via SSH, and executes a command line you configure, including custom paramaters associated with the alarm (things like Source IP, User, etc.). While it's possible to use the ESM itself as your bastion host, you run the risk of introducing a big performance impact if there are lots of alarm actions triggered.