Their kernel modifications and patches are public, and some of them have been upstreamed long ago. You'll need to compile your own to get the benefit, but I don't see why you wouldn't be able to use your kernel of choice.

Of course, if you want the native integration WSL offers, you'll need to upgrade the Linux driver/daemon side to support whatever kernel you prefer to run if it's not supported already. Microsoft only supports a few specific kernels, but the code is out there for the Linux side so you can port the code to any OS, really.

With some work, this could even open up possibilities like running *BSD as a WSL backend.