I believe it is as feasible to 3Dize a 2D movie as it is to colorize a black and white movie. If you remember when the first movie colorizations were done, they looked terrible. It was very expensive to do it by hand coloring each frame manually and humans did a poor job. But many of the more modern colorized versions actually look really good, often indistinguishable from a native color movie. Computers have made it relatively cheap and easy to colorize a movie with high quality results. The human identifies the color of various objects and the computer follows them across frames and helps minimize the amount of work.

The same kind of software would be needed to create a 3D movie from a 2D original. The humans have to queue the system by telling it which objects are at what depths and the computer can follow objects frame-to-frame and generate the calculated left/right views. It won't be as good as a originally 3D filmed movie because there won't be as much variation in depth of different objects. But I suspect it would be 90% as effective. A viewer would have to look closely to notice, for example, that all the objects sitting on a desk are in the same plane as the desk while the person standing in front of it is in another plane.

Generating a 3D version of a 2D movie will still be much more expensive than creating a blu-ray from an existing film which really just involves some touch up of the film and then rescanning with a high rez scanner. Unless the film is badly damaged (faded, scratched, etc) in which case making a good blu-ray release can be a major undertaking.