Smart Converter Pro provides two ways to add subtitles to a video.
Add your own SRT file
To add you own subtitles simply drag your .srt file to the 'drop here' spot as seen in the screen shot below.
Other subtitle formats
Smart Converter Pro only supports .srt subtitle files. If you have another format of subtitle file (e.g. .ssa or .ass) try out a free online subtitle converter to convert to .srt and add to Smart Converter Pro that way.
If you don't have your own subtitles you can use the search function as seen in the screen shot.