You do get a supply route option (war table mission) into the frostbacks early in the game. As you get more control in Orlais and Ferelden I can easily see food being supplied to/sold to the Inquisition.
Ram also seems plentiful just outside Skyhold. 
Premodern military supply didn't work that way. Overland food supply was paltry before railroads reduced transport costs; virtually all premodern armies had to be primarily supplied by the territory they camped in. That's why they'd make winter quarters in fertile valleys. Humping the majority of food - in addition to all other trade - over mountain passes would be ridiculous. Even movements on campaign were significantly dictated by supply: armies couldn't stay in mountains or deserts for very long, because the amount of food that could be carried with them was very very limited. Quartering on a mountain - above the tree line, no less - would be insane.
Even riverine and littoral food transport was costly, and doing it in large quantities was a severe financial strain on even some of the world's greatest powers. Rome spent insane amounts of money supplying its vast armies on the Rhine with British grain; the Chinese empires broke the bank building, maintaining, and expanding the Grand Canal partly so that their northern armies could have access to fertile Jiangnan. These were the
cheap options; by and large, supplying military forces with rations they were not able to gather themselves locally was out of the question until the advent of the railroad.
The point is, the setting isn't designed with military supply in mind, but rather with other entirely valid things, like rule of cool and "nobody really knows this stuff anyway so it's not worth incorporating it". So inferring other details about the setting based on real world analogy is silly; it should just be taken
prima facie. At least, that's my opinion.