Do you have the POLYSOLID command available in Civil 3D? If so, check it out. It is no problem to create polysolids with different heights. You just draw or select an existing line, having defined the height, thickness and justification, and a 3D solid looking very much like a wall will be generated. Later you can add doors and windows in 2D lines on a face of the wall, then use the PRESSPULL command to press the holes for them right through the solid walls.
If you already have a 2D drawing you can use PRESSPULL from that too, just go into an ISOMETRIC perspective in modelspace before you use PRESSPULL, the better to see and appreciate what you are doing.
You might want to watch this to help you get started.