To close them, either click on the first point again or press the Close button: We can give the walls a nice hatch pattern. Select both walls, make sure their Make Face property (located on the Data ...
Since FreeCAD version 0.18 there is a new C++ DXF importer, and since version 0.19 also a new C++ DXF exporter. These new components are installed with FreeCAD. To use the older, legacy, DXF importer ...