Live Events do not move to Stream on SharePoint. The live event technology will eventually be driven by Teams. I'm not sure what the future holds for Live Events in Yammer given this change.
Please ask Yammer if they have plans to support external live events in Yammer. As Kevin mentioned live events are moving to be powered by Teams. Teams does support external live events but how that would work when you are having the event in Yammer I'm not sure.