Convert srt and smi subtitles
Here's a very simple Ubuntu tool for converting between the two. It uses the command line.
First, install libsubtitles-perl by running:
sudo apt-get install libsubtitles-perl
Next, navigate to the directory your subtitle file is in, then use one of these two commands to convert:
SMI -> SRT
subs -c srt filename.smi -o filename.srt
SRT -> SMI
subs -c smi filename.srt -o filename.smi
And that's it!
I also discovered an online tool that seems to do the job: