Thats not possible in ISPConfig itself. But as far as I know, this can be configured in postfix. But I'am not sure if you can limit it to one domain. i dont have a howto hat hand, but you may find some pointers on postfix.org.
First please read those two links: Why not to add a footer Why not to add a footer#2(scroll down to 11) And now a solution: I found this email thread. and to restrict it to certain email address:
adding footer I believe, here is the link: http://howtoforge.com/add-disclaimers-to-outgoing-emails-with-altermime-postfix-debian-etch
