I could be wrong, but this is how I would try to do it.
In order to convert DVD, you would have to first Demux the PAL VOB's into an audio stream and a video stream. Reformat the video streams into NTSC format. Then remux the audio streams and video streams back to a VOB.
Sounds easy, right. Well, first you'll need to correctly break down the VOB's. The real trick here is to get your video streams into a format which your video editing software will read. Most Video Editors don't like the .m2v format so you'll need to convert it to a friendlier format.
Discouraged yet? It's only been three programs so far.
Once your video files are in an editable format, you'll need to open them in your video editor and save them as NTSC MPEG-2 files. Make sure you selcet to proper quality and resolution when saving the files. Now just convert the files back to .m2v (DVD video stream).
Using yet another program take the new video streams and the original audio streams and convert them into new VOB's.
Your done right? No, sorry. :(
I can't even begin to tell you how to convert the DVD menus into NTSC. (Only a real issue if the menus use full motion backgrounds)
Once all of your VOB's and IFO's are created just burn the new DVD using the DVD-Video option on your software.
Each of these steps require you to have a relatively advanced knowledge of the software used and a willingness to learn by trial and error.
In many cases, it would be well worth the money to simply buy the NTSC version of the DVD if possible as NTSC (American) DVD's usually cost less than their PAL counterparts.
This should be used as a guideline only, read additional threads to learn the specifics of such a task.
Just a note, I have never attempted this type of conversion so I may be completly wrong. I'm American and my wife is Japanese, both NTSC countries.
