Include Package Assets in their own bundle

For example, TerrainLit, TerrainLitBase and TerrainLitAdd shaders are all added in my root scene and then added again to every other scene that uses terrain. I would like to put them in their own bundle so they can just be downloaded once rather than bloat all the different terrain scenes. I'm able to do that with normal shaders in the Assets section and it saves us a lot of space in our bundles.

Right now I can't see any method for adding a Package item manually to a bundle. The UI for it just doesn't show up in the Inspector (like it IS there in the example below). Is there a way to do that? Or is that a new feature I'm asking for?

Thanks!