Baking from cubemap to LightProbes

I want to light my scene with 9-component irradiance, the main values of which come from my skybox.

Unfortunately, I can't google any ways to do this, because all the google hits on "unity", "irradiance" and "skybox" are dominated by Marmoset Skyshop. I've got nothing against Skyshop (and I own it), but my assets already have loads of textures on them, and I'd really like to just use 9-component SH and some ALU ops for this. And I'm on mobile and only need the diffuse term. And I use all my own shaders, and I'd really rather just enable per-vertex SH lightprobes than drag the entire Skyshop shader infrastructure into the project to do something so easy.

So:

1. Does Unity lightprobe baking bake the skybox directly to the probes?

2. If not, I can use D3DXSHProjectCubeMap() to generate 9-component SH from the skybox, and then directly set those values to LightProbeGroup?

I'm really surprised there isn't an easy sample of this? Maybe that's because it's so easy that you just assign your skybox to a cube in the scene and bake, and I missed it?

Thanks, Alex