Adding subtitles using mplex, or any other utility

Sirs

I have been using mplex to "stitch together" m2v and mp2 files into an 
editable stream.

First I use Projectx to clean up a Humax .ts file, then I use mplex to 
multiplex them back together. A typical command line I would use is

mplex -f 8 -o file.mpg file.m2v file.mp2

This works a treat, unfortunetely, however, I now need to add subtitles 
[file.sup and  / or file.sup.IFO]

Please, what is the best and easiest way to do this?

Thanks in advance

Dave Hansford

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