Is there a way to make a spiral "ramp" to easily model a spiral staircase?
Yes, but not in AGi32. You'd need a different modelling program to do so. Whatever program you ultimately used would need to either export out into *.dwg, *.dxf if importing directly into AGi32.
Another possibility is SketchUp, which can export files out via the Lighting Analysts plug-in to be imported by AGi32, but I don't believe SketchUp has a standard spiral option. It does have an extruder function that follows lines, but you'd have to get a spiral line for it to follow. There is a very good chance, however, that someone has already made a plug-in for SketchUp that does exactly what you want it to do.