To add a multi engine rating to a private, commercial, ATP, or CFI certificate, the FAA requires an instructor endorsement and a practical test. A Knowledge test (written) is not required. The practical test includes a detailed oral test.
Pilots may take their original private pilot or other practical tests in a multi-engine airplane, in which case they will be subject to additional experience requirements. A pilot certificate obtained in such a manner will not include single engine piloting privileges (ability to deal with a total power loss is not demonstrated during multi engine certification).