When units camp in the field they automatically scavenge a small amount of food. If a unit has a high logistics skill they can scavenge more than they consume allowing them to replenish their supplies.

So to answer your question, units can't really replenish a city because they need to be out in the field to camp and scavenge.

As a warning, a unit's view distance drops dramatically when camping and they are much more vulnerable to attack for about 30 seconds after they break camp.

The scarcity of food in the north was intended to add the challenge of managing food when campaigning in locations more remote from the map core. At the risk of making the game too hard or too dependent on logistics, perhaps we made food more plentiful in the southern areas than we should have.

Admittedly, camping and foraging was a feature that didn't prove as useful as we originally hoped. We made a few changes in Gold to try to make it more useful but it still wasn't effective enough to justify its inclusion.

I don't know if you've read the PC Gamer article on the Rome announcement, but the camping/foraging system is a good example for the last section of the kind of sub-optimal feature we decided to trim to make room for better and more useful features.

So in Hegemony Rome we've overhauled the system completely so that now constructing a camp when you first move into an area to campaign will be key to a successful strategy.

We'll be getting into some of the new strategies for Rome in the coming weeks/months but camps in Rome also replace the fixed fort system from PoM/Gold so in addition to using them to construct siege equipment and resupply your troops you'll also use them to secure your borders and establish footholds in enemy territory.

As with the sandbox mode in gold, you will be able to do things with the non-roman factions in the Rome sandbox that aren't historically accurate but make the gameplay more balanced and interesting.