You need to combine two tools:
Complex Profile, and
Magic Wand.
For your steeple, define the profile with a fill in the Complex Profile tool.
Set the wall tool to use this profile.
In Archicad, the easiest way to make a symmetric, multi-sided shape is with the magic wand in a circle.
Draw a circle to the diameter required. Set the magic wand to the number of linear segments required.
Magic Wand the wall to the circle and, VIOLA, there's the dome.
Go play.
Dwight Atkinson