Extending the email wftask merge tags

[Guido1982]

I want to create some functionality: when a comment is placed, we send e-mails to the person mentioned in the comment. In that e-mail, I'd like to show the rest of the conversation. To that end I'd like to extend the existing workflow merge tags to accomodate a new tag that gets and prints the conversation that preceeded this comment. How would I start extending this functionality?