Short answer: Not directly. Long answer: Direct voyages via sea to Germany would of been impossible because of the British blockade. Ships heading to neutral Scandinavian nations were often searched. However, one could could of taken a ship to Norway, travel overland to Sweden, and then hop on a ferry across the Baltic. Another way an American could get to Germany would of been traveling to France and then going into neutral Switzerland and then to Germany. However this would not be a good approach given that France was at war and you may of been searched or questioned. Unless you were willing to travel to Germany via Asia, Oman, and caravans there isn't another way to get there. Germany was fighting a two front war against major powers with naval dominance. The blockade was so tight that even Sweden suffered from food shortages in 1918 despite being neutral! So yes, an American citizen could travel to the Kaiserreich in WW1, but one would have to go through several different countries and likely would be searched.

It would be worth noting that soon after the conflict started American journalists like E. Alexander Powell were filing reports from German-occupied Belgium, and the US ambassador there tried to keep Nurse Edith Cavell from being shot and also would have to work with the Germans for US famine relief to the population. The Secretary of the American Legation, Hugh Gibson, published his journal of his time there, which is a great read.