Yes it can but it might be difficult to do in a dynamic fashion and I'm not sure if it would be fast enough in real time? The reason being it is really an iterative process where you start it, add the objects, then end it.
NDB_StartCompoundCollision to have the wrapper initiate the array of objects
NDB_AddCollisionToArray will add the collisions to the array in the wrapper.
NDB_NewtonCreateCompoundCollision to get the complete collision reference.
It might be possible if you don't clear the final collision, that you can use that to do you first
NDB_AddCollisionToArray and then your second call would be the other object you want to join. I've never tried this but it might work.