You can't. All you can do is enable mail aliasing also OR disable it and have another server handle email for that domain. That requires making mx record changes for the domain alias on whichever box DNS is hosted on.
If I understand the issue correctly, one possible workaround is enable a mailbox in the parent domain. Any mail sent to the alias which has a mailbox in existence on the parent domain will therefore receive email sent to the alias.
I may not have phrased that too well, but maybe an example will clarify:
parent domain: 123.com
domain alias: xyz.com
desired mailname on xyz.com: "
[email protected]"
approach 1: create mailname (with a mailbox) on 123.com called "bingo" & email sent to
[email protected] will flow into that mailbox.
approach 2: create mailname "bingo" on 123.com with NO mailbox, but with a redirect to another 123.com mailname, OR an external redirect
user@some_other_domain.tld
Given that your question was a bit sparse and lacking a description of what you wanted to accomplish, I've taken a Scientific Wild Assed Guess (SWAG) at your goal. That may not be in fact what you want to achieve, so...ummm...errr "Your Mileage May Vary"
In any event, hoping this was of some help.
Regards,
/porky