How can I import subtitles/captions into a letterboxed video like this?
You can use ffmpeg. Make your subtitles with Aegisub or manually with a text editor. ASS subtitles are fancier, but SRT are simpler if you want to manually make them.

Alignment &amp; Margin

The default settings will likely work, but you can control the general location, or alignment, using the "keypad" coordinates. For example, 2 is bottom center. For more granular control you use the vertical margin. In Aegisub these options are available in Subtitles > Style Manager > Edit.



You can also choose the font and font size in this screen.

ffmpeg examples

Now run ffmpeg with the subtitles filter to make your hardsubs:

ffmpeg -i input -filter_complex "subtitles=subs.ass" output


Or if you want to avoid using Aegisub you can use the force_style option:

ffmpeg -i input -filter_complex "subtitles=subs.ass:force_style='Fontname=Arial,Fontsize=20,Alignment=2,MarginV=10'" output


Resulting in:



If you do want to use ffmpeg make sure to output from Premiere using a lossless format, such as UT video, because ffmpeg will re-encode it which is required when filtering.