This was brought up in a post in another forum. The HowTo above will basically let you add pseudonyms under the virtual domain, while Mike wants admin@domain.com and admin@virtualdomain.com to be different accounts. The problem is that "admin" is an account on the system. Whether it's using domain.com or virtualdomain.com is irrelevant, it's mail for the "admin" account on the SME.
With that info, you could set up administratorguy@virtualdomain.com to point to user fred. This would let you create an admin-ish email address for the user designated as the v-domain admin...