The short answer is you can't directly import any DVD or Blu-ray Disk information into any video editing program.
You could use the screen capture ability within the program to record the playback of such a disk while playing it in a playback program like VLC Media Player or use a third party program like OBS Studio to produce a file you could them import into the program. But primarily video editing programs are meant first and foremost for editing camera video footage and now to some extent screen captured screen footage from things like game play but that also has its own problems.
Alternatively you could use a program like DVD FAB to rip the DVD onto your hard drive and then convert and edit onto a watchable video.
I'm not at my Magix computer at present but IIRC, for unprotected disks, if you pop in your DVD and try to drag it onto the timeline from the Media pool, MS will ask to copy the DVD to your hard drive, and then after that has been done, it will import the whole movie to the timeline for editing. You can't edit the movie unless it is on a hard drive in your system. If you've already ripped it, the same procedure applies using the files in the VIDEO_TS folder on your HDD.
After you have edited the movie, you can then reauthor the DVD using the burn tab.
I have, in the past, modified individual VOBs do go back into the DVD but it's a bit of a fiddle as you have to use the "Use/Encode all" options in the Burn process.