If you're copying a DVD, you might have luck if you try ripping the ISO, and then exporting it as and mp4 using something Handbrake, but if the video file comes as is and you're trying to merge the two, it might be a little more complex.
It would also depend on where you're sourcing the video from. I've had to 'burn-in' the titles which is fine for what I want, but in your case probably not. From my many hours of trying to do this, it would seem that simply adding the srt to the same directory doesn't work. I have a number of films in mp4 format which are in my iTunes library, and have experimented with A LOT of options and work-around's. I'm yet to find a method that works every time for this issue.
