Bring it in as an object, then explode. That will place all of it on one - ArchiCAD - layer.
Alternatively, when you bring it in using Open, it is a group. Open it in ArchiCAD first, select all, Edit selection set - place everything on one layer dedicated, delete all the other layers, check duplicates, do the Consolidation, save as PLN and merge to your ArchiCAD file.
Take good care that the layers and other parameters of a DWG are handled in different ways when you XRef, when you Place as External Drawing, and when you Merge or Open. The explanation in detail was done by Thomas Holm, and you can find it in Tips&Tricks I seem to remember?
Djordje
ArchiCAD since 4.55 ... 1995
HP Omen