Creating a bag file out of a mp4 video

I have a .mp4 format video and want to create a bag file out of it to use for ORB_SLAM2. I know that need to publish my video to a topic. Once my video is being published need to record the video topic. I already create the image_transport package and the publisher node. Also build it. Its working but I need to publish a video stream topic not single image. There is tutorial code "Adding video stream from a webcam" but I already have the video steam. So any help regarding this? Thanks