Now how to have HDRI maps in P5: The trick is to decompose it to LDRI maps in HDRShop and recombine them in the material room. In this case, I exported to 8bit TIFF files from HDRShop, one at an exposure of +2 apertures, one at -6 apertures. The multiplication factor of 2^8 = 256 (8 aperture steps difference) brings the maps back to their original brightness.