Southeast Asia Partnership Expansion Act This bill expands a Department of Defense (DOD) program that focuses on improving cyber readiness and cooperation with the military forces of certain Southeast Asian countries. DOD currently has authority to conduct a pilot program to (1) enhance the cyber security, resilience, and readiness of the military forces of Vietnam, Thailand, and Indonesia; and (2) increase regional cooperation on cyber issues between the United States and these countries. The bill expands this program to include the military forces of the Philippines and Malaysia, extends the program through 2029, and specifies that the cyber enhancements are to be defensive.