Ambient light problem with custom sky

So I’m using hdrp, and I made a custom level editor. Every level theme you can choose from has its own global volume, with its own HDRI sky. This has worked out incredibly well for me so far. Heck, the ambient light is pretty much taken care of for me, based on the color of my skybox.

But now I have a CUSTOM level theme, with a sky that imports from an external image file. And I can’t do that with an HDRI sky, apparently there’s no function to do that, so I rendered the skybox on the inside of a sphere instead. Now my problem is that because I used some alternative method for rendering the skybox, there’s no ambient lighting.

How can I add some regular, non sky based ambient light? I’ve been looking it up and playing with all kinds of settings and can’t find a solution.