for 'converting' such to a six single images, you just need to split it, the kmaketiles tool can be used for that,
e.g. when the cube size is 1024:
Code
kmaketiles cubestrip.jpg split_%v.jpg 1024
(for a horizontal cubestrip use %h instead of %v)
then the conversion from normal six cubes faces to a spherical image can be done by using the "Convert CUBE to SPHERE" droplet or the kcube2sphere command line tool,