Anyone know how to send a message to everyone on a chat network?

You would have to be opered on that network to send such a widespread message. If you are, ask your Admin how to do it.

Please note that the only time you should ever message everyone on a network is for important announcements (i.e. server B is being rebooted in 5 minutes, if you are on it, please connect to server A instead) or other such things that concern the network.