You need to create your own class, for example a child of SGraphNodeK2Base.
In this class you can create any visual.
You need the UpdateGraphNode function, it describes the widget tree from which the node is built.
I recommend using the Widget Reflector tool to quickly understand what elements the node widget is built from and how they look.
Then override the function CreateVisualWidget in your UK2Node and return SNew(CustomSNodeClass).