I think the subtitle thing is pretty easy (relatively speaking), although I've never done it myself. You'll need to do a little work in a subtitle editor, and then prep it for DVD authoring. You're basically adding one subtitle (a black box), positioning it in the right place (upper right corner), and then telling the editor to turn in on at frame 1 and turn it off at the end of the film.

As for authoring, I hear very good things about DVDlab -- but I don't know if the bargain-basement version will do subs at all. Might have to get Studio or Pro. Can't remember.

Authoring means you'll need all the assets from the disc you've got: audio (as either MPEG or AC3), video (M2V with no audio is best), menu pages (as PNG, I guess?), and your sub file. DVD Decrypter should be able to easily demux from the disc you've got, IIRC, even though there's no encryption to be Decrypted.

That should get you started. Do a bunch of reading (and learning) on your own, and you'll know more about this than most of us here -- me included. Go on: be the 'one black bar subtitle' guru!