Apart from not having any emergency exits, having looked at the 747 freighter you have three separate areas, all without windows and all a maximum of about 5 foot 6 high (at best) internally. I would suspect that if you fitted these out with seating there would be a fair reduction in this headroom.
In your favour is that these are all heated and pressurised, but have no in-flight access routes. However, the complete lack of reference points would be a severe impediment to anyone unfortunate to be in them during flight. Just think how uncomfortable it feels to be in a lift with a broken indicator panel when it is in motion, and that is in a protected environment accelerating in only one direction, now think how unpleasant iot would be to be in a sealed chamber accelerating in three dimensions. This would be an effective recipe for a true "vomit comet".
Why, other than to challenge the number of seats in an A380 would you want to put seats into these compartments?