You can use subtitles and texts saved in .txt files and produce a .stl file based on EBU-standard teletext format. This can be done if programs like SoftINI Subtitler Suite. However several file formats use the stl extension so you may have been looking for some other conversion.
Convert Simple text file to EBU-standard open and teletext file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert txt to stl on Windows
A multipurpose subtitling and captioning solution for HD & SD
Convert from txt file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.