If your core geometry is complex, you could create the 3D geometry of the foam core in SpaceClaim and then create surfaces on top and bottom faces of the foam core:
You can select "Share" for the share topology setting to have conformal mesh between the solid core and the surfaces. Without "Share", you need to define bonded contacts between the foam core and the surfaces (used to define the carbon layup).
Then, in Mechanical, you can mesh the model and create named selections:
In ACP, you will only see the top and bottom surfaces used to create the composite layup:
But, when you link ACP pre to a Static Structural system, the foam core will show up in Mechanical:
Let me know how it goes.