Currenty there is no way to add subtitles to videos on DEVONthink.
But I figured out that there is a way to soft encode subtitles to videos via terminal. Source: HRH George J Rickle IV's answer to What is the best free (no watermark) subtitle adding software? - Quora
In short, I should run following command after replacing X,Y and Zs according to instructions:
ffmpeg -i XXXX.mp4 -i YYYY.srt -c copy -c:s mov_text ZZZZ.mp4
XXXX: mp4 name
YYYY: srt name
ZZZZ: output mp4 name (since DT allows files with same names in a group, my preference is making this same as original mp4 name, ie XXXX)
The problem is, I have hundreds of video-subtitle pairs, and running this code manually would take enormous time.
It would be great if I select a pair of video and subtitle files and run a script via toolbar, and see output file in the same group. Can someone please share the script or lead me through the way how a AppleScript novice like me can achieve this?