Hi Anja,
To convert them to objects for use in ArchiCAD you will have to open them up individually and save each one.
Do this by going to 'File'>'Libraries and Objects'>'Open Object', then select 'Browse for a Folder', select dwg as the file type, then select the file you want to convert. Save the object you have created and it will now be available to place on the floorplan using the object tool.
You will need to repeat this for each dwg file, I'm afraid, as I don't believe there is a batch conversion method for doing this.
Good luck!
