1. Here's how

The guidelines below assume that you have got access to Adobe Photoshop or a similar image editor.

1.1: Render an uncompressed image in .TGA, .PNG or .BMP format:

1.2: In Lumion 8.5 and older, please disable any effects that change the contrast, saturation etc if you want the cleanest possible Custom Output Maps:

• Chromatic Aberrations Effect.
• Selective Saturation Effect.
• Noise Effect.
• Bleach Effect.
• Analog Color Lab Effect.
• Bloom Effect.
• Vignette Effect.
• Color Correction Effect.
• Sharpen Effect.
• Material Highlight Effect.
• Blueprint Effect.
• Tilt shift Effect.
• Depth of Field Effect.
• Volumetric Sunlight Effect.
• Handheld Camera Effect: Radial Gradient & Film Age.
• Water Effect: Underwater.
• Cartoon Effect.
• Manga Effect.
• Oil painting Effect.
• Outlines Effect.
• Painting Effect.
• Pastel Sketch Effect.
• Sketch Effect.
• Watercolor Effect.
• Exposure Effect.

1.3: In Photo Mode or Movie Mode, render a Sky Alpha mask in .BMP format:

Photo Mode:

Movie Mode:

Sky Alpha map example:

1.4: Add a duplicate of the Sky Alpha map to a new layer in the still image in Adobe Photoshop:

1.5: Select the Sky Alpha mask layer and in the top menu, select:

Select  -> Select Color Range -> Fuzziness = 200 -> Click on the black part - > OK

1.6: Hide the Sky Alpha mask layer.

1.7: In the top menu, select:

Edit -> Copy Merge

1.8: In the top menu, select:

Edit -> Paste

1.9: Hide the Background layer:

1.10: In a new layer below your cut-out building layer, paste a sky photo of your choice and adjust it as desired: