Yes, it is possible to convert blu ray movies to dvd-video format. You will need a blu ray drive of some sort to read and rip the movie to the hard drive first, and some sort of decryption program for blu ray.
Use a free program called BD Rebuilder for the conversion. You'll see two links in my signature regarding this program. The first covers the basic setup for the program and the second deals with conversion to other formats.
You'll get better quality compressing to double layer dvd size, but even single layer dvds made from blu ray can be quite good when using BD Rebuilder.