I searched and a few posts hint at it, but there are no explicit solutions that I can find.
I can't see it happening.
You can do it with forums because you control the database.
The sequence is
1) Add users
2) Add topics
3) Add posts
You're blocked at 1). You can't add the users.
A long time ago I re-created a couple of Yahoo groups on Yahoo but I was only able to do it by posting as a single user
i.e. all posts appeared to come from list-owner@groupname
Sorry but there it is.
Cheers,
Wilson.