Opacity attribute in my material


I’m just discovering Omniverse, and so far it is awesome!

I builded a simple USD scene inside Houdini, just a closeup shot of a sand dune with some animated sand grain blown by the wind. The texturing is done inside Substance Painter and the materials created and assigned through Create.

It renders just fine but I would like to edit the opacity of my grains as they take off. My idea was to create an opacity attribute and I to feed it to my instances material. I cannot see a way to do this from the UI, how should I approach this?