there are 2 types of underdumps: 1) underdump with zero bytes (you can freely add 2048 bytes of zeros to the end of the file) 2) underdump with missing data (you'll need to redump a game with the right tool once again... or wait for a fix from anyone else)
use the 1st method with NBA_2K12_PSP-VENOM and you'll get a perfect dump ;)