DivX Converter can include SRT, SSA/ASS, and DVD/XSUB*
After you have added your video to the converter, click the "+" button on the subtitles section
Browse to your subtitle file and click "Open" to add the subtitle file to conversion queue
Now click "start" to begin converting your file. Your encoded file will now have subtitles.
* DivX Plus profile does not support rasterized DVD/XSUB subtitles