This is typical behaviour when you export your model from another 3D programs - they all are converted into objects (as ArchiCAD generates GDL scripts to represent geometry). If 3D model, exported from external application, needed only for reference but not for future change in ArchiCAD - it works quite well. To improve drawings quality (for example to show right cut or change building material) it is possible to explode these objects into morphs, that later can be modified - like to choose different classification, change surface colour, cut lines end similar. It is also possible to edit geometry, but geometry editing will work quite similar way as Sketchup - it will require more manual input then just stretch the standard ArchiCAD wall.
The only format, that can convert wall into wall and slab into slab is IFC.