Put that in your houdini pref folder to create a Null connected to the selected node if you click Shift in the graph editor (houdiniXX.X\python2.7libs\nodegraphhooks.py)

nodegraphhooks.py

import hou
import canvaseventtypes


def follow_flags(parent, child):
    if parent.isDisplayFlagSet():
        child.setDisplayFlag(True)
    if parent.isRenderFlagSet():
        child.setRenderFlag(True)


def createEventHandler(uievent, pending_actions):
    if isinstance(uievent, canvaseventtypes.MouseEvent) and \
       uievent.eventtype == 'mousedown' and \
       uievent.modifierstate == canvaseventtypes.ModifierState(alt=0, ctrl=0, shift=1):
        selected = hou.selectedNodes()
        mousepos = uievent.editor.posFromScreen(uievent.mousepos) - hou.Vector2(0.5, 0.2)
        if len(selected) == 1:
            parent = selected[0]
            child = parent.parent().createNode('null')
            child.setName('null_' + parent.name(), unique_name=True)
            child.setInput(0, parent, 0)
            child.setPosition(mousepos)
            follow_flags(parent, child)
            return None, True
        for parent in selected:
            child = parent.parent().createNode('null')
            child.setName('null_' + parent.name(), unique_name=True)
            child.setInput(0, parent, 0)
            position = parent.position()
            position[1] = mousepos[1]
            child.setPosition(position)
            follow_flags(parent, child)
        return None, True
    return None, False