I think directly booting an existing Linux with WSL2 will be either impossible or really tricky.
But what you can definitely do is mount your existing disk under a fresh WSL2 install https://learn.microsoft.com/en-us/windows/wsl/wsl2-mount-disk . You could then try to chroot into it maybe. But don't get your hopes up too high. WSL integration with GUI apps is okay but not comparable to a native experience.