You can't "extend" an enumeration.
That's because all simple type derivations are restrictions--not extensions.
You can use union types to extend enumerations:
<xs:simpleType name="Sizes"> <xs:restriction base="xs:token"> <xs:enumeration value="small"/> <xs:enumeration value="medium"/> <xs:enumeration value="large"/> </xs:restriction> </xs:simpleType> <xs:simpleType name="ExtendedSizes"> <xs:union memberTypes="my:Sizes"> <xs:simpleType> <xs:restriction base="xs:token"> <xs:enumeration value="extra small"/> <xs:enumeration value="extra large"/> </xs:restriction> </xs:simpleType> </xs:union> </xs:simpleType>