You'd have to download a 3rd party launcher.
Did anyone notice in the settings for S Voice there's an option to wake up the device from sleep by saying "Hi Galaxy"? I tried it and it works pretty good, but I'd be more interested in getting to Google Now, not S Voice.
I read somewhere, you can chain "hi galaxy" and then the command to launch Google and then "Ok Google". Phone isn't mine so i can't test but it should work.
"Hi Galaxy that goes to Google Now would be awesome.