You know that DVD’s do not have the same quality as a majority of the professional formats available, or you would not be asking the question. Sure, DVD is not the best quality, but if that’s all you have to work with then it will have to do. Some DVD’s are better than others, a professionally made disc with proper compression will look better than something made with a home DVD recorder, so you best bet is to choose your sources carefully.
If you want a program to copy the contents of a DVD onto you computer to be used as editing material, this is a favorite of mine. Squared 5 makes a great freeware program called MPEG Streamclip (https://www.squared5.com/). This program works just like the capture window in FCP, with the ability to set in/out points. The program uses any of the codecs you already have installed in your system (which should give you plenty of good options if you do have FCP installed). Once these files are created, they can be imported and used normally. The compression codec that you use is important, I have seen a noticeable difference between exporting in DV50, ProRes and 8-Bit Uncompressed, all from the same source DVD. Do some tests to find the codec that works best.