You don't need to setup the email accounts on the domain aliases. After Mail service is enabled for aliased domain under Domain Aliases, each email account on the primary domain gets alias in the aliased domain. firstname.lastname@example.org, for example, will be able to send/receive emails as email@example.com. So, to create a email address in domain alias, just configure the alias to have mail services enabled and create the email account on the primary domain.
Please advice, if you have any further questions or concerns.