You can export it as a *.3DS file from AutoCAD, then import it into ArchiCAD with the 3DS importer.
By default the 3DS importer is not loaded into ArchiCAD.
You will need to go to your tools menu----->Add-Ons manager----> Search for more add-ons-----> then brows to your goodies folder, which can be found in your ArchiCAD folder.
In the goodies folder you will find a folder called 3DStudio in.
After the initial loading of the add-on, ArchiCAD will need to be restarted, you will receive a message after you have loaded it.
Once ArchiCAD starts up again, you will find the 3DS importer under your "Extras" menu.
HTH.
Cheers.
Ben