After a bit of experimentation, I found a free program that can do the same thing as DVDneXt Copy. It can build a blu ray movie for you from many different dvd-videos, or even other sources. It is called EasyBD Lite 1.1.
It is not as easy to use as DVDneXt Copy if you are using dvd-video as input, since it wants demuxed audio and video streams. It can accept other types of files as input, so it is not as limited as DVDneXt Copy, but the audio and video streams have to be compliant to blu ray specifications beforehand. It will not convert them or adjust them in any way. EasyBD Lite can accept .sup files as subtitles, but it is very picky about them.
I used five dvd movies as input, demuxed them with PGCDemux, and brought them into the compilation. There is an option to have each movie play, then go to the top menu, but this doesn't work at all in the free version of this program. There is NO TOP MENU. So you have to set the movies to play in sequence.
This program isn't a great choice I'm afraid. If you have only one movie you want to put into a blu ray structure, you can use tsMuxeR, and it is simpler to work with.
And putting two or more dvd-videos back to back with no way to jump through them or select each movie is not very appealing. Chapter points are lost, so the only way to go through quickly on a stand alone player would be to fast forward.