what you need is alternate identities each account
you can set by cos if a user is allowed to have any alternate account adress (he can enter whaeverhewant.whitehouse.bla)
or only bound aliases
in both cases you have to make a new identity or edit the from address of the default ident
in case they are boudn user can only use the dropdown to chose any aliase on main domain AND samealaises@otherdomain
in addition you can set a basic action for those accounts like "reply to",.... action but you need a new ident or edit default
people can choose then always which address shold be used when they are wrinting new or replying
i think its possible to set new identities by batch zmprov - but sorry i dont know the exact path for this
ps: do not forget you can also set the signature depending on the account -
for example signature 1 for idetitity default
signature2 for identity
anotheralis@otherdomain.com