Yes, you can send messages from within the dashboard to the discussion board in the app using the ‘Host Message’ button.
Host messages appear with a black background in the app and in the dashboard, making them easily visible.
You can create a Host Message in any of the message views in the dashboard. The above image shows a Host Message in the 'Published' view but if you have Moderation turned on, you can also create a Host Message in the 'Reviewed' message view too. Why would you want to do this? Well, if you have a presenter they can be looking at the 'Reviewed' message view, looking at the messages you are adding here but if you need to tell them something important: "You've got 5 minutes left" then as a Host you can quickly send something which stands out from the rest.