Lewis and Clark National Historic Trail Extension Study Act of 2007 - Directs the Secretary of the Interior to conduct a study to determine the suitability and feasibility of extending the Lewis and Clark National Historic Trail to include the Eastern Legacy sites (the sites associated with the preparation or return phases of the Lewis and Clark expedition, commonly known as the "Eastern Legacy," including sites in Virginia, the District of Columbia, Maryland, Delaware, Pennsylvania, West Virginia, Ohio, Kentucky, Tennessee, Indiana, Missouri, and Illinois).