You first have to specify the custom packages load path and then add the name of the package to your scheme system configuration.

For example:

You have a scheme file in /path/to/guix-custom which includes the declares linux-nonfree.

You run `export GUIX_PACKAGE_PATH=/path/to/guix-custom` and now guix will be able to find this package.

You use the module and include in the declaration

`(kernel linux-nonfree)`

PM and I will share my configuration :)