As the war went on there was a significant reduction in the availability of horses to transport supply (especially in Germany, but also in Britain) which caused the powers to rely more and more on lorries and trucks to accomplish this task. The ability to create these units is not reflected in the game. As far as I can see the only trucks are those which accompany the US forces when they enter the game. (Incidentally I have seen photos of hundreds of German military trucks lined up to be turned over to the allies as a result of the Versailles treaty and roads crowded with British lorries carrying American soldiers and supplies to the front.) I believe that beginning at least in 1916 Truck units should be available for construction on a limited basis for all the powers while Horse drawn Supply Wagon units should be automatically replaced by Truck units a few at a time each turn beginning in 1916, similarly to the way German heavy artillery units are replaced earlier in the game. However, automatic conversion should never replace all the initial Supply Wagons or any of those captured during the game.
Introduction of truck units should reduce the supply requirements of these units and those they support due to the reduction in forage required by the horse drawn units they replace. Which was another reason the various armies tried to reduce their dependence on horse drawn units. I have read the fully half of the tonnage shipped from Great Britain to the continent during the war was forage for horses.