First, there are many who will tell you that feeding raw isn't advisable at all. I'm not one of them.
By "grain free" are you referring to wet of dry?
In either case it isn't a matter of just adding something to a commercial processed food and, viola, you have the equivalent of raw. The commercial pet food manufacturers already add back the basic nutrients that get destroyed during processing. Unfortunately they also add a bunch of other stuff that isn't part of a cats natural diet (grains, fruits, veggies, artificial colors and flavors, etc) and what they add back that is part of a cats natural diet is better obtained in it's natural form from raw meat.
Take a look at this site. Dr. Pierson explains it better than I can:www.catinfo.org