I have now a Raspbian Stretch configured according to my needs, with also other software therefore I cannot use the Volumio 2 image.
How can I get Volumio 2 to work in it? what should I download, what are the steps for installation?
I think you should provide Volumio 2 as a package that any Debian based distro can install. If someone wants the kernel from your distro, or wants ease of installation, can get the whole image. All the other ones can just install the package on top of their installation.