Hi--my thought is that custom food sharing is unavailable because Fitday cannot vouch for the accuracy. However, I don't know why they cannot add it as an "at your own risk" venture.
As an aside, I feel that Fitday, overall, could vastly improve the foods they offer information on--one of the biggest reasons I have so many custom foods is because of that. I believe Fitday probably uses an officially sanctioned list of foods, but is it upgraded? Can more be independently added? Every food you can purchase has the nutrition info. on it unless it is fresh, and yet a great deal of these foods are missing.