That is a bit complex and depends on how you have it configured. Try checking the Howtos at postfix. http://www.postfix.org/docs.html
It is very dependent on how you have configured your mail system.
if you love man pages ... http://www.postfix.org/postconf.5.html
look for virtual and forward ...
from memory, if you have something like this in your main.cf:
virtual_alias_maps = mysql:/etc/postfix/mysql_alias.cf
alias_maps = hash:/etc/postfix/aliases, mysql:/etc/postfix/mysql_alias.cf
you can just define a new alias in /etc/postfix/aliases and re-run newaliases (make sure it acts on the right file though).