Installing a custom package on Guix binary setup

[Vicente Vera]

Hello,

Currently i'm using a Guix binary installation on top of my GNU/Linux
distro. Setup went fine.

So far i've installed only substitutes (everything OK), but there are
some packages that I want to tweak a little.

How can I install a customized .scm package definition?

For example, I want to pass some configure flags to Emacs (such as
'#:configure-flags '("--without-dbus" ...)'). I copied
/gnu/store/dbf9i96ac2pbyaibg591bx90a8zhvngc-guix-latest/gnu/packages/emacs.scm
somewhere else, did some editing, but now I'm not sure what to do with
it.

Going through the mailing list i've found out that I might need to
build Guix from source to do such a thing, but want to know if it can
be done with a binary installation.