I would look at the nss-certs package and make a package definition using
nss-certs as a template or even inheriting from it.

I have now had a quick look at nss-certs and it is too complex for me to
change without sitting down with it for a while. So I think I would then go
on to use the simple build system or what it is called.

Because what you need is basically copy a file to somewhere your browsers can
see it? To learn those paths I'd still look at nss-certs.
I see              (let ((certsdir (string-append %output "/etc/ssl/certs/"))
in there. That's probably your path.

To use this definition with guix: put it in for
example /home/username/guix-modules/certs.scm and then add this directory to
your $GUIX_PACKAGE_PATH. I do the following in my .profile
export GUIX_PACKAGE_PATH="$HOME/guix-modules:$GUIX_PACKAGE_PATH"

I'm traveling and so might not respond quickly.

HTH
Tonton