Just to get 2 options out of the way
To create a 3d from a dwg or dxf you have to open as an Object, under File-> Libraries and Objects-> Open object or you can use the 3DS plugin if you export from Maya as 3DS.
Archicad does not do scripting so every solution will be a workaround. Another option to try is to export the "holes" from Maya as a solid object and them subtract them from the slab but it might not behave well. Another option is to export the 2d surface and use it as a template to "magic wand" the holes in the slab though it will take 2,500 clicks.
eduardo rolón AIA NCARB
Another of the forum moderators.
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