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)