I dig Guix, but Lisp is also one of the most commonly reviled syntaxes, so it's hard to say it's clearly better, or that Nix would enjoy more adoption with S-expressions.
Reviled by people who have never used it.
Most commonly and with lots ans lots of prejudice.
Is it possible to write a guix package that includes code which redefines the package object?
Yes!
http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/...
You'll see here that the package emacs-minimal actually inherits from the emacs package definition, and then all fields afterwards are essentially overwrites of whatever was in the emacs package.
Unfortunately, I just noticed that the manual alludes to this, but actually never explicitly documents it in the "Defining Packages" section :/
... http://gnu.org/s/guix
Lisp is pretty good at lists.
The only important language is the derivation language sent to the daemon. Nix spends so much time building up inputs... for shell scripts. I always felt like they would have been so much more successful if they chose some other, more popular language to generate derivation.