You are describing closed caption files, not open captions, aka subtitles. Are closed captions what you want? If so, they have very limited formatting options, which vary with the file type (eg SRT, SCC, etc). The way the captions are displayed is also very dependent on the player - your TV, QT Player, VLC, Switch, etc. I don't think any of these options include shadows or borders.
If you want to turn your .itt file into subtitles, you will need a third-party app, such as one of these from Spherico: