Meshing a for CFD Simulation in an air-filled cross-section

Hi everyone! 

I need your help! I designed a CAD model in ProE 5.0 (.asm): It's like a cuboid with an air-filled cross-section. The CAD model is build with 6 assembled plates (.prt). 2 side, 1 back, 1 front, 1 top and 1 bottom plate. Each plate has got 8 mm thickness. The attached image shows you the half 3D-model as a lateral cut.

For a following CFD simulation with AcuSolve I imported the .asm-model-file as geometry into HyperMesh (CFD). Just because the import into AcuConsole failed.
My focus is on the air-filled cross-section inside the 3D-Model. This is what I want to mesh! And define the surfaces as 'walls'.

I tried 2 ways to realize:

1) I did a 2D mesh of the surfaces and afterwards I tried to do a 3D cfd tetramesh. The problem is, HyperMesh doesn't want to mesh the cross-section...he only meshes the Solid between the surfaces of each plate.

2) I defined the plates as solid and HyperMesh meshed them as solid. But afterwards it's impossible to do a 3D cfd tetramesh for the air-filled inside...

Is the .asm-file with 6 parts (.prt) the problem or is there any other possibility to define the air-filled cross-section? Just to pick the inner surfaces is impossible because you'll get some free-edges  ...

Thank You for your advice!!!