Simply like this:

ffmpeg -i input.srt output.ass 

To list all the subtitle codecs that FFmpeg supports, you can type:

ffmpeg -codecs | grep "^...S" 

The output should look like something like this:

 ..S... = Subtitle codec DES ass Advanced SubStation Alpha subtitle DES dvbsub DVB subtitles DES dvdsub DVD subtitles D S microdvd MicroDVD subtitle D S pgssub HDMV Presentation Graphic Stream subtitles DES srt SubRip subtitle DES xsub DivX subtitles (XSUB)