When you open the DWG file, there is a Translator selected at the bottom of the dialog box. Click the Settings
... button, go to the Settings in Selected Translator panel > Drawing Unit
and change the Value of 1 DXF/DWG Drawing Unit in ArchiCAD
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.