Could the cargo hold of an aircraft be re-designed to carry passengers?
In principle, yes. The cargo hold is pressurised and (depending on the aircraft and the installed options) reasonably temperature-controlled. However, it's probably not feasible; there are three major issues to be worked out that I can think of off the top of my head:


Cargo hold height and windows. It's a bit hard to tell from your picture, but even on a large widebody plane the cargohold is not that high (the cabin floor is lower than halfway). The A330 (as an example) carries LD3 containers in its cargohold, which are about 160cm tall; the cargohold is not much taller than that. Most passengers cannot walk upright inside it; the cargo hold on smaller aircraft is even lower (the one on the A320 is designed to carry LD3-46 containers, which have a height of about 115cm). Also, the lack of windows would bother passengers in the lower deck. 
Emergency exits. In an emergency, passengers must be able to get out of the plane within 90 seconds. At best you would need to install additional doors in the cargo hold; at worst, you will not be able to meet this requirement (because of the low ceiling, people can't move as quickly) 
In a crash landing, the lower fuselage may buckle, which serves to cushion the impact for the people in the passenger cabin. You would need to show that the passengers in the lower deck can survive a crash landing. 


Note that some airlines use part of their lower deck for galleys, crew rests and even lavatories.