All thanks for your reactions so far.
I have done some googling myself:
2 KLM 747 Combi’s have been put in service again.
The Martinair MD11s were true convertibles.
E class fire suppression (By means of depressurization) is indeed not allowed in combi’s.
I don’t think a pressure resistant floor is technical feasible for the A380.
Gas/smoketight will probably already be a challenge??
Only Class A, B, (firefighting by flight engineer or additional crew) or C class (Halon)
The last is expensive and heavy.
The extra firefighters for A and B are already on board by means of the cabin crew.
but this will require additional space and walkways between pallets.
It will be clear an A380 combi will not load same as a full freighter 747 or 777 but how much pallets and tons of weight could probably be loaded on the lower deck?