Andrew,
The MaxonForm/C4D plugin connection is the most organic and two way one between Archicad and any other freeform capable software. Just going throught the Maxon tutorials is giving you quite a bit of information.
You can also, as many do, use SketchUp as a freeform or easy modeler, and then import the geometry or convert the components into not so parametric library parts.
Don't forget to try ArchiForma, and explore the more traditional ways of generating blobotecture in Archicad ...
Djordje
ArchiCAD since 4.55 ... 1995
HP Omen