Step 1: First, To add a dome to your sector, extrude a sector out of the side you want to dome at, and then extrude the new sector until you have serveral layers on top of each other.
Step 2: Multi-select all but the one sector you want to make a dome out of.
Step 3: Go into tools (F9 key) and then set the scale to 0.95. Now, click scale to scale the mutil selected sectors.
Step 4: As you see, the sector that was not multi selected had been created in a cone affect. Now, repeat steps 1 - 3 but, in step 3, set you scale to 0.1 less than the previous (if last was 0.95, then next would be 0.85). Once you have the dome effect made, then your done!