It's been a few years since I've used it, but the free MPEG Streamclip software can do what you want - it will pull the video off a DVD as an .m2v file, which is exactly what Adobe Encore wants. AFAIK, the video is not converted/transcoded, and Encore will not need to transcode the .m2v. Just build new menus and burn again and video quality should not have been touched.
I don't remember what the audio ends up as, but pretty sure about the .m2v thing. Probably been 6-7 years since I've used the software to be honest.