Dalius:
I'm not sure about the corner window, but custom shaped windows can be done, in the Help files see: Elements of the Virtual Building > Parametric Objects > Graphic Creation of Custom Library Parts and Components >
Saving Custom-Shape Doors or Windows.
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.
David