If you want to make the Xbox permanently NTSC, as in output NTSC and only play US games then you will have to do a little fiddling, but it is possible.
Either 1, mod the system temporarily, change to NTSC and Region using Config Magic, then unmod or...
2, using software mods. Easiest is probably to use a save game hack and boot up Evox, FTP to Xbox, send over config magic, patched with xbedump (too big for memory card) and load it up, change to US region and NTSC output and then delete config magic afterwards if you want.