If the dvd is not finalized, the pc drive probably won’t be able to read the disk. This is why the software you are looking for isn’t available.
You can finalize the disk, rip the contents to the hard drive, then build a new dvd structure and burn to another disk. This is certainly possible, but a lot more trouble than what you had in mind.
There are commercial products that can take dvd-video as input and let you make a new menu. VideoReDo TV Suite, the Womble DVD Wizard program, and DVD Lab Pro can all do this. (Lab Pro is $$$).
The free authoring programs would most likely want mpeg2 or even elementary streams as input. DVDStyler can import the vob files in your dvd-video, but they have to be merged first…so an extra step there. You could use Vob2Mpg to combine the vobs into one large mpeg2 file to import to the authoring program without loss of quality.