For a simple, fast, easy answer? No.
You’re going to have to create your own subtitles. Even if you already have, say, a .srt file, they’re not easily edited, whereas you’ll be doing wholesale gutting. Easier to just start over. You’ll need a subtitle creation tool. (Several of them out there, mostly freeware, all come with a learning curve).
If a tool imposes some limitations, you can live with that to get the output you want. By the way, exactly what IS that? If you’re satisfied with watching it on the computer, or on a DivX-compatible player, that’s one thing. If you want universal video DVD disks with switchable subtitles, it’s quite another.
You can do subtitles in a separate file, which imposes its own limitations. Or you can flatten the subtitles down into the movie, so they can’t be turned off, but you also don’t need a video player that can handle subtitle files. This means you need a mixer/converter, of which there are several, free, learning curve.
If you’re starting with a video DVD, it’s basically in MPEG-2 form. That’s old and inefficient, but it’s what must be used for video DVD’s that work on any player. Now if that’s also your desired output, it makes no sense to convert the thing to MPEG-4 only to convert it back to MPEG-2 again. DivX/XVid are MPEG-4.
On the other hand, if the tools you select work only/better with MPEG-4 files, and the dual conversion doesn’t harm the quality too much, you may prefer it that way. In any case, the one thing this won’t be is fast.
Find several subtitle creation tools, play with them, decide on the one you like best. Create your new subtitle files and make sure they work as expected. This is phase I, and necessary whichever way you go afterwards. Then make a definite decision about output, which will control what you do next.
Understand that this process is not easy. Subtitles are a lot trickier than they appear. Which frame, exactly, to the microsecond, should it start with? How long should it last? How big should it be? What typeface? Where in the frame? Are those last three static, or will they change? It’s a painstaking, detailed, prolonged and pretty boring process. If it were easy there’d be a lot more subtitled movies available than actually exist.
Since you only want to do a relatively few frames, you’ll have it somewhat easier. Somewhat.
As to the software, well, there are some things that just go together, like bread and butter, death and taxes, or free software and lousy/nonexistent documentation. This’ll be as much “teach yourself” as “practice”, and mostly by trial and error.
