Yes, it is possible to add a CustomBrush_Landmass in the Layer Blueprint Brushes by using your own Blueprint Actor (BP_Manager) via nodes. According to the Unreal Engine community forums 1, you can spawn a CustomBrush_Landmass using your own Blueprint Actor, which should also automatically add in the “Edit Layer Blueprint Brushes”.
Another way to add a CustomBrush_Landmass is to use the Blueprint Brush section in the Landscape Editor. According to the Unreal Engine documentation 2, you can select the CustomBrush_Landmass option from the Blueprint Brush section and add it to your procedural landscape.
I hope this helps!