The news buzz about the latest, large scale, cyber-attack generates more questions than answers. According to the latest reports on the attack at Target, and other retailers, the attack originated from a teenage hacker in Russia.
Using a known malware virus "kaptoxa", the coder performed some coding tweaks to the code and sold it on the hacker black market as a virus under the name "Blackpos".
Modifying existing viruses to develop new viruses is not really news worthy and no different then what other programmers do with other existing code. It is common for a coder to take existing code and modify it for their specific requirements.