Hi. I wrote expression for adding Hack font family to GuixSD. You can see
it my guix package repository:

https://github.com/8p8c/my-guix/blob/master/packages/font-hack.scm

Here is patch for fonts.scm:

--- fonts.scm    2016-06-02 08:01:49.387086500 +0300
+++ fonts.new.scm    2016-06-02 08:05:30.005373478 +0300
@@ -9,6 +9,7 @@
 ;;; Copyright © 2016 Nils Gillmann <niasterisk@grrlz.net>
 ;;; Copyright © 2016 Jookia <166291@gmail.com>
 ;;; Copyright © 2016 Eric Bavier <bavier@member.fsf.org>
+;;; Copyright © 2016 Dmitry Nikolaev <cameltheman@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -921,3 +922,54 @@
     (home-page "https://fontlibrary.org/en/font/fantasque-sans-mono")
     (license license:silofl1.1)))

+(define-public font-hack
+  (package
+    (name "font-hack")
+    (version "2.020")
+    (source (origin
+              (method url-fetch)
+              (uri (string-append
+                    "
https://github.com/chrissimpkins/Hack/releases/download/v"
+                    version
+                    "/Hack-v"
+                    "2_020" ;; FIXME
+                    "-ttf.zip"))
+              (sha256
+               (base32
+                "16kkmc3psckw1b7k07ccn1gi5ymhlg9djh43nqjzg065g6p6d184"))))
+    (build-system trivial-build-system)
+    (arguments
+     `(#:modules ((guix build utils))
+                 #:builder (begin
+                             (use-modules (guix build utils)
+                                          (srfi srfi-26))
+
+                             (let ((PATH     (string-append (assoc-ref
%build-inputs
+
"unzip")
+                                                            "/bin"))
+                                   (font-dir (string-append %output
+
"/share/fonts/truetype"))
+                                   (doc-dir  (string-append %output
"/share/doc/"
+                                                            ,name "-"
,version)))
+                               (setenv "PATH" PATH)
+                               (system* "unzip" (assoc-ref %build-inputs
"source"))
+
+                               (mkdir-p font-dir)
+                               (mkdir-p doc-dir)
+                               (for-each (lambda (ttf)
+                                           (copy-file ttf
+                                                      (string-append
font-dir "/" ttf)))
+                                         (find-files "." "\\.ttf$"))
+                               (for-each (lambda (doc)
+                                           (copy-file doc
+                                                      (string-append
doc-dir "/" doc)))
+                                         (find-files "." "\\.txt$"))))))
+    (native-inputs `(("source" ,source)
+                     ("unzip" ,unzip)))
+    (home-page "https://sourcefoundry.org/hack/")
+    (synopsis "A typeface designed for sourcecode.")
+    (description "The Hack Font is designed for the screen.")
+    (license
+     (license:non-copyleft ;;FIXME
+      "https://github.com/chrissimpkins/Hack/blob/master/LICENSE.md"
+      "Hack Open Font License v2.0"))))

But there are two FIXME issues. First with URI string generation. Sorry, I
haven't mastered Scheme yet:

              (uri (string-append
                    "
https://github.com/chrissimpkins/Hack/releases/download/v"
                    version
                    "/Hack-v"
                    "2_020" ;; FIXME. This should be generated from version
"2.020"
                    "-ttf.zip"))

And second with licence. I've marked it as non-copyleft, but I'm not sure
about it:

    (license
     (license:non-copyleft ;;FIXME
      "https://github.com/chrissimpkins/Hack/blob/master/LICENSE.md"
      "Hack Open Font License v2.0"))

Dmitry