the bad news is: you probably can't use them

The problem with those kind of sups is, that they are graphics and depend on the resolution of the video pic. I mean, even the same subs are different if the movie (of course the same one too

) ist broadcasted on SD, HD or released on DVD or Bluray.
This should not be the biggest problem, but alas, I don't know ANY sub converter that allows different input formats. You can find some for SD, I also found a good one for BluRay, but none of them accepted a different format than they were supposed to.
And, sadly, I found none so far, that was able to deal with HD TV subs (maybe the bluray thing works for 1080i too, but none accepted 720p for now).
Your MythTV does simply record what the stations are broadcasting, so it depends on WHAT Channel you are recording from.
But, as already told in the beginning: usually you don't find any program that will be able to deal with them.