Man the question is so easy.
I built a room with bsp.
Now I just want to Place a wall somewhere in the room.
I want to know how to place something in specific location.
For example 400 units far from one of the wall
I'd suggest using external modeling program and use static mesh. Modeling with BSP can be challenging. If you still want to pursue BSP modeling, I'd refer to 3D Buzz tutorial on BSP. I believe it's the Geometry Mode tutorial. Actually, I'd suggest watching all of the tutorials.