I once saved a campaign generated mission of the ODS 30AE mod and discovered that it somehow assigned a Transfer (not Transport) mission to a cargo plane. After further investigations, it seems it's a valid mission type that can be assigned to AI-only aircraft in ThirdWire's Mission Editor. Just add Transfer to the roles of the aircraft in its data.ini file, and it'll appear in the Mission Editor. I don't know the exact differences between Transport and Transfer, but on the campaign generated mission it seemed to make the aircraft take-off from one airport and land on another.
I want to reiterate that this is an AI-only mission type, so it won't appear for the player, regardless of the aircraft type.