Make it 3D after the fact.

Stereoscopic conversion or dimensionalisation is the process where 2D motion picture material is converted to stereoscopic vision.

The process is labor intensive because it requires skilled artists to create depth and make decisions that can not be automated. Although stereoscopic conversion can never produce the full volume look of a film shot with two cameras, the process can yield visually stunning results.

Stereoscopic conversion is an option for high budget productions only due to high cost. It is also a good option for library material.