Add border to video, maintaing aspect ratio, and insert subtitles in the border
You can use FFmpeg, a free command line tool to do this:

ffmpeg -i input.mkv -vf pad=1920:1080:0:140,subtitles=filename.srt -crf 20 -c:a copy out.mkv


The subtitles filter accepts ASS styling parameters, such as font size and margins.