If you simply want to make a mailbox receive mail for another address, just add the full address to /etc/postfix/virtual:

[root@srv-2 postfix]# cat virtual
usr-1@networking.com usr 
usr-1@networking2.com usr 
usr-2@maildomain.com anotherusr 

If we simply add this line to the bottom:

usr-3@maildomain.com anotherusr 

Mail that was was delivered previously via usr-2@maildomain.com will be delivered in the same way via usr-3@maildomain.com. After you change the file, run postmap:

[root@srv-2 postfix]# postmap /etc/postfix/virtual