I’m not sure how you would do this and retain the original menus. It should be possible to make a blu ray video with the main movie from each of the dvds included on the disk. You can make a new menu to select each of the movies. Blu ray specifications do allow standard definition on blu ray media, though it isn’t often seen.
If you want to try this, get a free tool called MultiAVCHD. It can output in blu ray format, as well as AVCHD and others. I haven’t tried this, so you might have to look carefully through the settings to find out how to retain the original encoding parameters, and not let the program re-encode to H264. I think I would get a BD-RE disk (rewritable) for testing purposes.