1). Best way is to send it to a dub house to be dubbed through a hardware standards converter.
2) if 1 is not an option, capture as NTSC, then use Compressor (Advanced Format Conversions) and choose a preset that matches your timeline codec setting. Be sure to turn on the Frame Controls in the inspector and set the resize and rate conversion to better or best quality. This can be a lengthy transcode , so test a short section to see if you are happy with the results.
3). If 1or 2 don’t work for you, purchase the Standards Conversion plug in from www. nattress.com
4) if all else fails, edit with Avid Media Composer v4.x, which can do this conversion (and all other conversions) quite nicely in the timeline.