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)
}}}