I use nix, but I assume the same goes for guix, you write a package or run it with something that emulates a FHS (like steam-run, don't mind the name) and hope it works.

Writing a package is not as bad as it sounds, nix and guix have very little difference between "package maintainer" and "user" so most users probably can package software. Normally copying something from nixpkgs or similar will get you most the way there.