You can generally import .txt text files to any subtitle generating programs and save them in some of the most common subtitle formats, including SRT. Some simple text editing changes (like typos, text encoding) can also be fixed by using any common text editor.
As for a ways to convert txt to srt on mac, you can find special subtitle editors for mac, just check Apple Store and same goes for subtitle app for Android. You can find some on Google Market.
Updated: July 29, 2022