How do I add an audio track from one DVD to another?

I have the R1 (NTSC) and R2 (PAL) versions of "They Live". R2 has an audio commentary track, R1 is at the correct aspect ratio and is NTSC.

I want to just add the audio commentary track from my R2 DVD to the R1 DVD. I don't even care if the PAL speed difference is corrected if that complicates it too much. Nor do I want to add an extra button on the menu or whatever for the track. Either I will select the new track manually on the remote or just remove one of R1's other track and put the commentary there instead.

So is this possible and is it difficult to do?