You could resort to calling zip in a separate process:

iex(1)> for file <- ["foo", "bar", "baz"], do: File.write(file, file)
[:ok, :ok, :ok]
iex(2)> System.cmd("zip", ["zipped.zip", "foo", "bar", "baz"])
{"  adding: foo (stored 0%)\n  adding: bar (stored 0%)\n  adding: baz (stored 0%)\n",
 0}
iex(3)> File.write("extra_file", "some more stuff")
:ok
iex(4)> System.cmd("zip", ["zipped.zip", "extra_file"])
{"  adding: extra_file (stored 0%)\n", 0}
iex(5)>
BREAK: (a)bort (A)bort with dump (c)ontinue (p)roc info (i)nfo
       (l)oaded (v)ersion (k)ill (D)b-tables (d)istribution
^C~ $ mkdir example
~ $ mv zipped.zip example/
~ $ cd example/
~/example $ unzip zipped.zip
Archive:  zipped.zip
 extracting: foo
 extracting: bar
 extracting: baz
 extracting: extra_file