>We need to add the user to the primary domain in order for them to receive
mail in the virtual domain.
No, you don't have to do it this way! Instead of a virtual domain,
create an alias for the primary domain. In IMail Administrator, go to your
primary domain and, under "Aliases", write as many other domains you may
want (separated by a space, up to a total of 255 characters, if my memory
serves me).
HTH,
Rubens
