You have to package your custom packages using guix. A good place to start is here
Also you can use flatpak (to install discord or Firefox) on guix just make sure you use --user flag.
Also I'm on the doom-emacs discord and a lot of us are guix happy guix users if you don't feel like hitting up the irc channel or mailing list.
Server run by the main developers of the project It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!