Hi,
If you want to add custom HTML after the "Additional information", you can simply copy/paste your HTML after the line:
<!--{ENDIF:ORDER_SUMMARY}-->
when you edit the email via the menu System>Emails.
To preview the change, the best is to try with the "order notification" email which you can preview by editing an order via the menu Orders and then clicking on the "email" button at the top. That will open a popup with a preview of the email to be send.
The template of the order notification emails are roughly all the same so once you have what you want for this one, you can just copy/paste the changes to the other notifications.