How to configure a domain alias for mail?

[aseques]

Hi,
I've two domains, example.com and example.org.
My customer wants that all the mailboxes from example.org have get redirected to their equivalents in example.com.
At the moment I see that this feature is not yet in ispcp.
How should i implement it in my server?
Basically I am worried that any domain rebuild could break what i've done, and I also would like to keep the reject in "smtp time" for unknown recipients.

My intention would be to keep a list like the one in /etc/ispcp/postfix/working/aliases, and I could generate it with a cron, but maybe there's something more neat.