Modify density of volume in geometry nodes

I have an OpenVDB volume. I wanted to apply a gradient to it, making it denser toward the top and less dense toward the bottom, before applying a volume to mesh node.

However, it seems there is no way to do this. We can create a new volume using a texture for the density field, but there's no way to change an existing density, by multiplying it with a texture, for example. Such a feature would allow volumes to be fine tuned and edited using geometry nodes.

This is related to, but different from, "Geometry nodes volume density field".