You are correct. Grouping *is* the heirarchy in AC3D.
You can still set the object centers per object; it depends on the exporter whether it will read the group center or the object center. I *think* most use the object center.
You can download some plug-ins here that will let you adjust the object centers numerically or move them to the position of the selected vertex:
http://independentdeveloper.com/arch...in_pivot_tools
Here's the heirarchy for a character I set up as an example, if that helps: