Yes, to create a wall, you have to create an instance of Wall class then add it to the Home instance returned by [font=courier new]getHome[/font] method in the plug-in you'll define.
For doors and staircases, you have to find the CatalogDoorOrWindow or CatalogPieceOfFurniture instance you want in the furniture catalog stored in user preferences, then use the catalog object to create a HomeDoorOrWindow or HomePieceOfFurniture instance, change its coordinates and size before adding it to the home instance with [font=courier new]addPieceOfFruniture[/font] method.
Objects added to home will appear in the plan and the 3D view accordingly.