There exist some dedicated subtitle editors that are used not only to edit or create new subtitles, but also to convert them between individual subtitle formats. So it shouldn't be hard to find one that can convert SRT subtitles to SSA (SubStation Alpha) format.
Convert SubRip video subtitle format to SubStation Alpha subtitle data.
We have found 5 software records in our database eligible for .srt to .ssa file format conversion.
Articles and other information relating to srt into ssa conversion:
Create subtitles for 3D movies - How to convert 2D subtitles to 3D subtitle file format with 3D Subtitler and Subtitle3d.com tools
Microsoft Windows software - convert srt to ssa on Windows
A free subtitle editor
Convert from srt file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned srt to ssa. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
An online subtitle converter
An online converter for subtitles
Multiplatform software - convert srt to ssa on any platform
A tool for creating and modifying subtitles
An older freeware subtilte editing application