You can drop the 6 cube faces onto PTGui. Switch to Advanced mode.
Lens Settings: set the lens type to Rectilinear and field of view to 90 degrees.
Panorama Editor -> Projection -> Preset -> Spherical Panorama Editor -> Blending side bar: Blender: 'Don't Blend'.
Image Parameters: Enter the yaw/pitch roll angles of each image (0, -90,
+90 etc).