Food miles is not based on scientific evidence - it is a myth, used as a protectionist/anti-trade measure. By all means, purchase locally if you wish to support local farmers but, in many cases, the TOTAL energy used (and CO2 production) to produce and ship food products from Southern Hemisphere countries to Northern Hemisphere countries is greatly lower than that of food produced locally.

The reason is that food products that are transported by ship use significantly less energy than any other form of transportation (note this doesn't apply to product flown in, which is actually very uncommon, due to the high cost and the ability to have produce ripen while in transit on a ship). You will use a lot more energy driving to your local supermarket to buy the product.

If you were to have fruits and vegetables produced year round locally, significant amounts of energy would be required to heat green houses or other structures for these products to grow in winter (or vice versa in southern U.S. States in summer). Despite the options you have proposed for this, the cost and energy used to build and run such structures is significantly higher than shipping fresh product around the world.

The other costs to consider are inputs. Generally, those products from Southern Hemisphere countries require less inputs for production, as their environments are more naturally suited to producing that product. In other words, less water, fertilisers, and pesticides are used, and free range pasture grazing (no barns) is used in the case of dairy and meat production (effectively eliminating the need for antibiotics, etc).