You can create subtitles from all kind of sources, even plain text files. However you will probably need to manually copy and paste the lines and edit them heavily using a proper subtitle editor tool. Some good choices for subtitle programs are listed below.
Convert Simple text file to SubStation Alpha subtitle file.
We found 2 software records in the database related to .txt to .ssa file format conversion.
Microsoft Windows software - convert txt to ssa on Windows
A free subtitle editor
Convert from txt file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned txt to ssa. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Multiplatform software - convert txt to ssa on any platform
An older freeware subtilte editing application