In case anyone else is following this, I subsequently found this information for "CINEMA 4D Exchange" in the Add-Ons Manager.
This add-on exports CINEMA 4D files with extra information for ArchiCAD objects, and merges back the modified CINEMA 4D data to GSM objects. So you can simply make renders with CINEMA 4D or create organic GDL objects for your project.
Locations in the user interface:
Design / Design Extras / CINEMA 4D Exchange
Open / CINEMA 4D File
Merge / CINEMA 4D File
Save as 3D / CINEMA 4D File
I also found this for the demo version of "Din3D File Exporter".
Din3D Exporter
Exports the 3D model (the geometry and the attributes) as Din3D files.
Support:
support@din3d.net
Locations in the user interface:
File / Din3D Exporter
Save as 3D / Din3D File