It is possible to make video subtitles (.sub files) manually from pretty much any text source with text editor program or specialized subtitles editors. Without such subtitle software, you would have to know the basic syntax of subtitle format (in this case SUB) and manually write the time stamps. This might be sufficient for some personal videos, but for longer videos / movies using specialzed software would be prefered choice for txt to sub covnversion.
Open TXT file Open SUB fileA multipurpose subtitling and captioning solution for HD & SD
Program for creating and managing movie subtitles