unless you're pretty familiar with DVD file structure, then it would probably be easier to get a DVD authoring program and just rip the DVD's audio/video streams, and then re-author it.

or, if you don't care about having pretty menus, you can use IFOEdit to author the DVD, easily. (and yes, you can insert chapter points).

to do this, rip the DVD's audio/video streams (SmartRipper or DVDDecrypter work fine), and make sure you have the ripping software set to "demux"/demultiplex the audio/video streams, and have it rip the audio/video as one long file for each stream (both audio and video).

then, at some point, find out where you want each chapter point... obviously the first chapter would be at 00:00:00:00. write down these chapter times on a piece of paper (or in Notepad or whatever), and use a video time calculator - there's a free one out there, which google will pull up i'm sure - that will convert time codes to video timecode. save this as a .txt file.

open up IFOEdit, click on DVD Author, then import the video and audio streams. under the "Scene changes / Chapters" section, there's a button to browse for/open up the chapter point text file you saved. select the destination (where you want the new VIDEO_TS and AUDIO_TS folders saved on your hard drive). then click OK, and it should start creating the DVD.

then just burn it with your software of choice. should work flawlessly. i know several people who only author DVDs using IFOEdit, as they don't want to take time to create menus.