If it's a system feature - as is the case on the PS4 now - all 2K has to do to figure out how to do it is consult first-party PS4 SDK documentation from SCEA. They don't have to talk to Sony San Diego at all to get custom music in, though they might reach out to them for some high-level pointers.
That said, the ability to implement custom music and actually scheduling dev time for implementing it are two different things. It's on 2K at this point to get us the features we want to see in the game.