It's a bit of a hardcore way, and lack of basic functionality in a web GUI like this I would say!
Anyway, what I did is:
- in Mailbox -> Custom rules, added 'redirect "bounces@localhost";'
- in /etc/aliases, added 'bounces: "|/some/script.sh"'
- created a system user "bounces"
- run "newaliases", so that Postfix picks up the new config
It would be great if there was an easier way to do it.