You can create a new video (same resolution and frame rate) with just text using the Text Source in OBS and then join the 2 using
MP4Joiner. If you just want to add text before a video, add a text source below the video source and hide the video with the eye icon. Record with just the text on the screen and and then show the video source by clicking on the eye icon. You really don't need a plug in to do that.
If you wanted to add the lower thirds on-top of an existing video, you would need to re-encode the video. This is generally not recommended.