Right now, there isn’t. But since we now have ODB++ (which supports the things you listed) export, there’s guidance how to represent such things.
In terms of UI, I’ve been thinking about extending the existing stackup dialog to support adding custom layers between existing layers. There’ll be some extra work required to fit them in and handle that their layer number doesn’t reflect the position within the stackup. A slight issue I see is that these new layers will have their colors set on a per-board level, so they might not work well across multiple users using different color schemes.
Out of curiosity, what’s your use case for these extra layers?