How would you change 25fps UK Pal video into a DVD that would work with a TV in the USA?
The confusing hurdles are the USA format NTSC, frame rate and region type for a DVD player.
My understanding is that Region Codes don't apply to home burnt DVDs.
As for changing PAL to NTSC I would check first whether the recipients of your discs can play PAL discs.
If you do need to provide NTSC then re-export your project as NTSC. If your source material is PAL expect some quality loss due to the 576 to 480 line conversion and change in framerate.
In my experience (I have a son who lives in America and have converted several PAL home videos for him) if you set the render template to NTSC at a suitable bit rate, Movie Studio does a very good job at conversion to NTSC. I've found that the default options in the NTSC render template are sufficient although some may advise tinkering with the de-interlace setting to deal with the extra frames that are inserted. More knowledgeable contributors on this forum may have further advice to add.
The only change i make is to disable resampling. This means that ever 5th frame will end up being duplicated, but it's barely noticeable. With resampling left on Vegas will blend frames together resulting in a ghostly doubled image, which i find very objectionable.