I worked on the analysis of the wing for the A380F - in fact it was my first major job in engineering. It was designed to a different set of loads to the passenger variant to represent a different fatigue spectrum and flight profiles which better suited the freighter business case.
The Freighter specific variant didn't make it because it couldn't operate in to enough freight specific airports as they stood, and those airports seemingly didn't want to expand. The 747 could.
I guess there's potential to retrofit cargo doors on the passenger variants, but maybe only one at each end.
I suspect that these aircraft will become niche lease for max passenger capacity periods only.