There are no other official channels, but a bunch of people have their own channels with additional packages. The most popular is probably nonguix (https://gitlab.com/nonguix/nonguix) which provides some popular nonfree packages.
Creating a recipe is generally easy (it's just a bunch of metadata) and even easier if it's supported by one of the importers (python, perl, emacs, ...). You can use guix on any other Linux system if you're not ready to install the complete system. The only difference is that your system is not managed by guix, and you can't run "guix system reconfigure", but the experience is otherwise the same. Make sure to follow the additional steps from the manual.