Allow non-admin users to join conference before admin

We have enabled secure domain, it fits almost all of our needs except this one:

  • we want non-admin users to be able to create/join the conference before admins (but not as a moderator). When an admin enters the conference, he will become its moderator.

How can we change prosody or jicofo configurations to achieve it?
Or do you think can we write a lua module to handle it without changing the source codes?

Thanks in advance