Is it possible to convert a passenger A380 to carry cargo?
Airbus originally planned on a freighter version and UPS and FedEx both ordered some of them. Production delays with the passenger version resulted in the freighter version being delayed and both of them canceling their orders.

The main issue why there is no freighter version is that the internal volume of the aircraft is too large to make it economical for cargo operations. I.e. the density of the cargo generally is too high to take advantage of the full internal volume before hitting the aircraft's maximum take-off weight. As a result, you end up hauling a whole lot of "extra airplane" around. The only time that extra volume is useful is for really low-density cargo loads. 

That extra internal volume is useful if you have to move a large number of passengers into slot-constrained hub airport like London Heathrow or JFK but packages don't care much about personal space...

In words other than mine:


  An A380-F would be too fat to fly at a profit: The plane would hit the maximum payload (a constraint of weight) before its maximum cubic space (a constraint of volume). Its design can’t support the maximum payload required to generate a profit.


Here are some good other more in-depth analyses. 

Something that was surprising to me initially was the amount of modifications that are necessary to convert a passenger aircraft to a cargo aircraft. This paper from a Bedeck Aviation Group (Boeing affiliated passenger to cargo conversion house) provides a good breakdown of the steps required for a 767 conversion. This video from Boeing provides a good visual summary as well (music and hair styles are an added bonus).