How 2D to 3D conversion is performed?
They've done it via rotoscoping techniques in order to create depth information. 

From fxguide.com about Titanic 3d Conversion:


  The team at Stereo D used no projection or automated solutions. The team isolated with multiple layers of roto each of the key elements of the scene, often many per person in the foreground and then these were used to derive a depth map. From the depth map normally a new left and right eye was created. 


Also see the wikipedia article about 3d Stereo Conversion and The Art of Stereo Conversion.