I don't think the forum is the right tool for this -
a forum is a discussion area, you write a post onto the forum and the email is effectively a notification to subscribers of that post. A forum is not intended as a means of communication with individual users, so there would be no mechanism within the forum to add a username as a mail-merge type field.
It may be possible to hack the core code that sends those emails to include a username prior to (or after) the main message, but that would affect all forum emails and not just the one you want to send in this way. I don't think there would be a way to include a 'mail-merge' type field within the body of the message as that would come directly from the forum.
You may want to see if anything like this is possible with something like the Quickmail block perhaps?