Is there an easy way to add a video as 'first-play' or intro (such as a trailer) to DVD files already on the HD or is it a re-author jobbie?
Never done this before...I assume the DVD files are .vobs? If so, I believe DVDlab[?] can do this. There could other tools for the job, but as I said, I've never done this before so can't help further...sorry.
Tmpgenc DVD Author can import VOB files directly.
You can also use SmartRipper to demux the VOB files to video & audio stream and re-author in Maestro, DVDLab, etc making sure you point First-Play to Movie1.