WASHINGTON, May 1 (Reuters) - U.S. authorities said on Thursday that former executives of collapsed energy trader Enron Corp. (ENRNQ) conspired with investment bank Merrill Lynch & Co. (MER) to pad Enron's balance sheet in a scheme involving electricity power barges in Nigeria. In announcing expanded charges brought against the former executives, the U.S. Justice Department said the executives worked with Merrill "to fraudulently improve Enron's balance sheet by illegally 'parking' poorly performing Enron assets." The assets, consisting of power barges moored off the coast of Nigeria, were the subject of extensive Senate investigations some months ago. The Justice Department...