Assuming not copyright protected material, you would need to "rip" the DVD disc by copying the movies contained in it and converting them to Mp4 format to import into iMovie, edit, and share out as an Mp4 movie that you can import into iDVD (or other DVD authoring program) and burn to a DVD.
You will need:
DVD-R single layer standard DVD
An optical drive DVD burner (either internal to your computer, or purchase and external burner)
The free download, Handbrake, to convert the DVD disc movies.
A DVD authoring program like iDVD or Toast, or the free one called Burn.
To begin, insert your DVD into the DVD player of your computer or an optical external drive. When the DVD's icon appears on the screen, click on it. You will see a screen that looks like this:
Open the VIDEO_TS folder (ignore the AUDIO _TS folder -- you don't need it) and you will see something like this:
Your home movie is contained in the files with the VOB extensions. (Ignore the other ones -- they are just data and back up files.) Option-drag (Copy, not move) the VOB files out to your desktop, making sure that the originals stay in the folder. You can then open those VOB files with the free download, Handbrake, and convert them to MPEG-4 videos that you can open and play, or import them into iMovie to make your compilation. Then you share out that movie to your desktop and drag it into the project window of an iDVD project. If you are using some other DVD authoring program, then drag the movie into whatever uploading window or procedure is required by the program. Then from the iDVD menu (or from some other authoring program) save the project as a Disc Image. From there you can burn the DVD by first inserting a DVD-R disc into your burner, and then clicking on the disc image icon and selecting Burn DVD from the drop down menu. Burn at 4x speed. And you are good to go. You can burn as many discs as you want from the disc image.
There are other ways to copy a DVD, but I have found that the quality is substandard. The best way is to work with the actual movies themselves. Also, if you are running HIgh Sierra and intend to use iDVD as your authoring program, you will find that it is rather quirky. Most of the themes, particularly the animated ones, do not work well, if at all. I use the theme called Full Frame '05, that works and is easily adaptable.
All looks very complicated, I know, but if you follow the steps it will work.