Emergency Running Message

Is it possible to insert an emergency message directly to the video while the video is still playing without closing the video?

If the video in question is a discrete file and not a feed from a RTSP/RTMP Server… short answer - No.

The video has to be Real Time… Per the documentation:
Videos uploaded to the CMS are cached for off-line playback on Displays.”

What this means is the CMS sends the video ahead of time to the Display Client.

To use a streamed video from a RTSP server, you must have rights to access it and the path to the video. Then use the “Local Video” media type.

Add Local Video
Video Path ___________________________________________
A local file path or URL to the video. This can be a RTSP stream.

A video being streamed from Your RTSP server is under your control. To post a message on the video (or superimpose it) is up to the software you are using on the video server… and what is creating the live feed. There are programs available to capture a video feed (live or canned) and super things on it. Outside the realm of Xibo though.