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)
  Last modified 13 years ago   Last modified on Apr 22, 2013, 11:59:25 PM  
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