There are no ready made tools for this conversion.
It may be possible to rip the Blu Ray disk to the hard drive using AnyDVD HD, then use EAC3to to extract the audio and put the video into a mkv file. But you then have to frameserve the video using an AviSynth script and feed it into an mpeg encoder to make an mp2 file. This could be done with HCGUI I believe, or ProCoder or one of the TMPGenc encoding programs.
Once you have the separate AC3 audio stream from EAC3to and the mp2 file, you can import to an authoring program and build a dvd. And hope that you have no audio/video synchronization problems.
Not simple. And the main decryption/ripping tool, AnyDVD HD, is not free. There is a free trial available for it. The rest can be done with free tools: EAC3to, AviSnyth, HCGUI, and a free authoring program like GUIforDVDAuthor.
Very few people are doing this conversion as yet, so the tools haven’t been developed. And protection on Blu Ray is robust, so much so that there is only one good decryption tool available.
It would be easier to buy or rent a few copies of the dvds.