Last year /u/thefourthmaninaboat and myself wrote answers to a related question on American-German trade during the Great War which you might find interesting.

The gist of these answers is that German merchants did indeed have access to American markets before the United States joined the conflict in the spring of 1917. It is very difficult to measure the scale of this trade, but it is clear that Germany was able to access to acquire substantial amounts of supplies and commodities from the US and the rest of the Americas, and that many of these goods would reach Germany by way of transhipment through neutral European territories.

The Entente powers were fully aware of this rerouting of German transatlantic trade, and made concerted, if not always successful, efforts to prevent it. As the war progressed the efficiency of allied economic warfare nevertheless increased dramatically. The German government also found it increasingly difficult to secure financing in the United States (rendering it unable to pay for its purchases). Combined these issues had by late 1916 effectively reduced German access to American produce to negligible levels.