import bpy
from bpy.props import PointerProperty

def init():
    bpy.types.Material.luxcore = PointerProperty(type=LuxCoreMaterialProps)

class LuxCoreMaterialProps(bpy.types.PropertyGroup):
    node_tree = PointerProperty(name="Node Tree", type=bpy.types.NodeTree)

>>> material = bpy.data.materials['Material']
>>> material.luxcore.node_tree
bpy.data.node_groups['Material']