Instead of downloading the kernel under work, I want to reuse an existing kernel source to help testing a hacked kernel for development.

I have implemented that at: https://github.com/cirosantilli/runlinux based on this repo. Feel free to link from the README to it if deemed worthy :-)

Also if you find it interesting, I can patch this repo to do that as well: we could add a --src flag that points to the desired source to use.