Email aliases in Gmail app
Proper aliases, where you can pick either address as the sender, isn't possible in Gmail. However, in your case I understand you never want to use ab123456@domain; you always want to use first.last@domain.

In that case, when adding a new account, use first.last@domain as your email address and choose manual setup. Enter your password when prompted and then, in the incoming server settings, change your username to ab123456@domain. Enter all other settings and do the same for the outgoing server.

Now you have email setup for your account that will always use first.last@domain when sending email. It will of course read email sent to either.