Yes, that would work.  I tried something like the following:

--8<---------------cut here---------------start------------->8---
(list (channel
        (name 'guix-chromium)
        (url "https://gitlab.com/mbakke/guix-chromium.git")
        (branch "master"))
       (channel
        (name 'guix)
        (url "https://git.savannah.gnu.org/git/guix.git")
        (branch "master")
        (commit "1d4c2dabefdc5e957ed74a58066f8faa8e0fb70e")))
--8<---------------cut here---------------end--------------->8---

Is there a way to inherit or modify %default-channels to avoid all the
copy-pasting?

Also do you know how to get the last guix generation commit?

-- 
Pierre Neidhardt
https://ambrevar.xyz/