Either way would work to update the ER-5 config.
If you're using an intermediate editor, make sure it'll handle unix-style line endings (e.g. notepad++ if you're using windows).
You'll need to reboot the ER-8 once (to let the thing load the base config -- sometimes they hang up on "load config" commands when they have no config), then you can just get into config mode after bringing the ER-5's config over.
configure load /config/config.er5 compare (add eth5,6,7 again) commit save exit