1. Use DVD Decrypter in IFO mode to demux the separate video and audio from the R1 disc, and the audio track from the R2 disc.
2. Use BeSweet to transcode the R2 audio, slowing it down to the correct speed so it will be in sync with the NTSC video.
3. Use DVDAuthorGUI to author a new DVD containing the video and 2 audio tracks. (However, you will lose the menu structure and any extras on the original disc.)
If you want to keep the disc structure, it can be done using a program called VobBlanker. Basically you multiplex a new title containing the two audio tracks, and this program will insert it into the disc structure in place of the existing title. But I seem to remember it might be a bit more complicated than that. Anyway, have a go a steps 1 and 2, post back here if you have any problems, then take it from there.