Berryessa Snow Mountain National Monument Expansion Act This bill modifies the boundary of the Berryessa Snow Mountain National Monument to include the Walker Ridge (Molok Luyuk) Addition, which is approximately 3,925 acres of federal land administered by the Bureau of Land Management (BLM) in Lake County, California. The Department of the Interior must administer the addition as part of the monument. Interior and the Department of Agriculture must jointly develop a comprehensive management plan for the monument in accordance with, and in a manner that fulfills the purposes specified in, Presidential Proclamation 9298 of July 10, 2015, relating to the establishment of the monument. The BLM or the Forest Service must enter into agreements, contracts, and other similarly cooperative and collaborative partnerships if requested by an affected federally recognized Indian tribe regarding management of the monument pursuant to the relevant federal authority.