Girish,

Install the plug-in CopyAsNewFurniture (see PDF).
Draw your tapered wall element(s).
Select them and use Tools --> Copy as new Furniture to copy and paste your model.
Delete the original and use 3D view --> Export as OBJ format.
Now use Furniture --> Import furniture.
During importing rotate your model into the correct position.

As long as 90 degree rotations are OK, this should work.
You can also have a look at the Roof tutorial (PDF ).

Hans