In a nutshell, creating a current roster update involves adding missing players and making the latest transactions so that the rosters reflect the real NBA teams. Updating the ratings for better and more realistic player performance has also become a key part of the updates. The only external tool you really need is DB Commander to modify hidden player attributes and other game data stored in .dbf files. A lot of the actual updating can take place within the game itself.
If you have those files readily available, it's a matter of placing them in the render folder and making some changes to players.dbf so that the game recognises that a player is no longer created and loads his face file accordingly. As far as creating the art updates themselves, I'll have to leave that to someone else as that's not my area of expertise.