I wrote this in Great Article on NPHC Sororities
thread.
"I think that there ought to be more communication among all groups, NPC, NPHC, NIC, IFC, and the multicultural groups. We have many of the same concerns, problems, interests, and goals. I think that our similarities outweigh our differences. I think that through increased communication, we all can better understand and learn from each other and work together. I do think it is important to respect our differences as individual organizations and conferences. The NPC was an idea that seems to have worked well and has lasted for over a century. Maybe there will be another umbrella organization for all greek letter organization in the future."
To answer your question, I don't know if there has ever been an all greek convention or conference. I wouldn't be surprised if something like this did happen one day in the future. We as greeks should work together while respecting our differences, and a conference might facilitate that.