Hello,
If you have root access, you could make a script to add them manually to:
/etc/virtual/domain.com/passwd
in the format:
bob:asdf123
fred:hjklasd
where the 2nd parameter is the crypted password.
If you don't have root, then you can make a test user backup with "POP Accounts for all domains". Download the file, figure out the format, insert your own emails into the file and restore the backup.
John