Hi Shokan, hi folks.
After you import the DXF file into your SketchUp (SU) model, provided that it is not sloppy cad, ie the edges are properly connected at the corners, there are no gaps at corners, the edges do not cross at corners or protrude inside a space that you want to fill, there are no overlapping edges, you can simply retrace any edge to close a face. Then you can push/pull it at will. There is also a utility that comes with SU that allows the closing of faces. Make sure that, in preferences, you enable all extensions. Then, in the Tools menu you shall have the choice Utilities --> Create face. Notice that you can create only one face at a time with this.
There are plugins that will allow you to speed up this process as well as plugins that may help in repairing sloppy cad files.
Just ideas.