there's no way for us to support that - behind the scenes, we'd end up having to do multiple auth attempts for every account, and keep track of when failure means failure, and when it means try again.
That said - you may be able to fake this:
Create all users under the default domain.
Add aliases under the domain you want them to appear to be at, with the same username.
Set the canonical address (for outgoing mail) on each account to reflect the address you want the account to appear to be under.
Then everyone can log in with their username, and mail will be sent to (and appear to come from) that user.
The only thing you lose is the display name in the web client - that will be for the
account@defaultdomain.com.