Expanding Access to Inpatient Mental Health Act This bill allows states to receive federal Medicaid payment for psychiatric and substance-use disorder services provided in institutions for mental diseases (IMDs) to patients who are enrolled with a Medicaid managed care organization (MCO) or in a prepaid inpatient health plan (PIHP). Current law generally prohibits federal payment under Medicaid for services provided in IMDs for individuals under the age of 65. However, states may receive federal Medicaid payment for monthly capitation payments to MCOs and PIHPs for services provided in IMDs to enrollees aged 21 to 64. Such services must be provided for no longer than 15 days per month and in lieu of other services covered under the state Medicaid program.