Your best option would probably be to add the hostmask manually, using .+user and .+host, rather than using the .adduser command.
As for automatically created records, you might be able to achieve something like this using "common userrecords". For this, you need to create a dummy record with a hostmask matching any user that would use a common userrecord (something like *!*@somehost.com). This dummy then needs the c userflag (common), and no further flags. See ".help common" and ".help whois" for further information.