Press W, then S for Sector mode. Hold the mouse over sector 2 (the north one) and press Ctrl+C to copy it, then press Ctrl+V. Move the cloned sector to the east side of Sector 1 (which we'll call S1; this new one is S4). Then create a 256x256 sector on the east edge of S4. Press L for Linedef mode, right click and hold the right-hand side of this new S5. Drag it to the right and slightly up, increasing the length of the room and skewing it slightly. Texture it.