Are there any regulations preventing one from converting an originally cargo aircraft to ferry passengers?
Converting a freighter to an airliner would require lots of work: 


you have to install an HVAC system capable of supporting hundreds of passengers, which means installing air ducts from the engines to the cabin, etc. Major surgery to the wing and fuselage.
you have to install a sufficient number of doors and emergency exits (with associated systems). Major surgery to the fuselage.
big changes to the electrical system to support e.g. entertainment for the passengers.


So the conversion would require taking most of the aircraft apart.