Is it possible to store subtitle srt files in an m3u playlist file?
m3u playlists only contains links to the media itself that you're playing.

It doesn't actually consider the srt file at all. If the software used to play back the m3u file support srt files it will look in the same folder for the same file as the video only with the extension switched to srt.

You will therefor always need to store the srt together with the video file.