Converting NTSC to PAL or vice versa is fairly time consuming and not all that simple most of the time. One commercial product that says that it can do this is ConvertXtoDVD.
A free all in one tool that you can try is AVStoDVD. No guarantees on success though, and you will lose menus and extras. http://www.videohelp.com/tools/AVStoDVD
It is usually easier to find a dvd player that can handle both formats. In fact, most PAL players can do this anyway.