The Blu-ray spec includes legacy support for standard def video; you often see such as Blu-ray extras. I've never made Blu-ray from scratch so I don't know what the tools would be.
DVDFab will author Blu-ray from a variety of inputs, including DVD, but I suspect it upconverts to hidef. Maybe not damaging, but unneeded.
As said, I would burn the videos to DVD DL (Verbatim is what I use).
Or: recompress with Handbrake. At q=18, "High Profile", the quality on DVD looks very good to me. Typically a 2:1 space saving. You will be playing MKV or MP4 media files then, not DVD Video, so it depends on your player.
Edited by wmcclain - 10/31/13 at 4:27am