Is there a way to allow a user who is subscribed to a list with an email
address, for example, [log in to unmask] to send postings to the list from this
address as well as from [log in to unmask]

It seems to me that I've seen a switch that can be set to let the list be
more
[snip]

I don't know of a switch (if there is one I'd like to know, too). I always
subscribe all their addresses and set all but one to NOMAIL.