The Machine Simulation in NX can be set up as G-code driven simulation, which means that it takes as input the posted code that will actually go to the machine tool. So yes, G code from any CAM system can be used to drive the Simulation in NX CAM.
The Virtual NC Kernal (VNCK) is a particular optional component of Machine Simulation, with which it is possible to plug in the actual Sinumerik controller software as a black box machine driver. Machine tool builders sometimes use this capability to create very accurate simulations, even including the Controller interface. VNCK is not required for G code-driven machine tool simulation and is not typically used by most customers. It is typically used by machine tool builders, as described.
Derek Hart ... if we're not having fun, we're not doing it right!