You can carve a hole into the wall using the carve tool (right click + carve) but don't do it with any shapes other than normal cuboids.
Build all the parts AROUND the hole out of brushes, like this
ascii code
+---------------------------+
| |
| brush 1 |
| |
| |
+---------+-----+-----------+
| | | |
| brush 2 | | |
| | | brush 3 |
| | | |
+---------+ +-----------+
...then I got some ap, and shot a big ass lazar at everyone.