Hi,
I know what’s in the image may seem obvious to you, but to me, with no frame of reference, that image is extremely vague; I have no idea what I’m looking at.
That said, you can place hidden objects to cover your walls (you’ll likely want to set Collision to “No Collision”).
Just make sure that the blocking mesh is casting a “Hidden Shadow” and that it is “Hidden In Game”:
Again, this is assuming I’m understand you and your photo correctly, the latter of which is immeasurably abstract.