There's no easy way to do this.
When an address is set to bounce, in most cases, our MTA simply sends a 550 error to the sender's MTA during the SMTP transaction; thus, it's the sender's MTA which actually generates the bounce message, and the content (other than the error message) isn't determined by us.
aura% telnet mx1.swarthy.mail.dreamhost.com smtp
Connected to mx1.swarthy.mail.dreamhost.com.
Escape character is '^]'.
220 ludo.dreamhost.com ESMTP
250-AUTH LOGIN PLAIN
550 firstname.lastname@example.org: User unknown
It would theoretically be possible to put some text after the 550 error (instead of the usual 'User Unknown', but we don't have any way to do this, nor are there plans to add this as a feature.
In some cases, a fake "bounce" message is generated (by a simple perl script), but it's not configurable.
You could possibly do what you are trying to do with an autoresponder, and almost definitely using procmail and / or Perl. Just be careful in either case, as you don't want to accidentally create an infinite loop of any sort.
Anyway, there's no officially supported way of doing this, and there isn't likely to be in the future.