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>