I have manually added .abr (Photoshop) brushes but they do not appear in the brush settings menu.
From the manual:
I'm using GC Version 10.7.5 (3483)GraphicConverter now supports cusom brushes for painting. Open the palette for the brush settings with a double click. In the bottom section you can select the preintalled brushes from a menu. You can add own brushes by copying the files into the folder Library / Application support / GraphicConverter / Brushes.