I brought that wall forward to prevent it.
But anyways.
Use a cubic (if desired) "Build Brush" (red) then position it in the desired location. Hold Ctrl and click (left/right) and move the mouse. Once positioned, press Ctrl+A to Add. Rebuild the geometry, then Raytrace.
If you need any more specific info, let me know.
I'll stick a copy of my fixed Ghetto Map onto
http://www.mobileforces.org/files on the 2
nd of next month (Feb) If you wanna copy of it.