Here's your start:
From SketchUp:
Export… as 3D Model… 3Ds. format
Into Archicad:
From the Help Menu, find the "Archicad Downloads" or "Goodies" link. Download "3D Studio In" and install this plug-in in the Add Ons folder.
Restart Archicad. Find the 3Ds importer under "File Special"
Use it to bring the exported SketchUp file in as an Archicad Object. Naturally you'll save it to a folder in the existing Archicad library to simplify loading.
I'm working a lot in SketchUp since becoming Western Canada's Certified Trainer. What fun. It turns architecture into a Video Game.
Dwight Atkinson