XmlSchemaSet fails to add xsd file with xs:override element

The example.xsd file contains this fragment:

<xs:override schemaLocation="../base/definitions.xsd">
    <xs:simpleType name="MessageTypeEnum">
        <xs:annotation>
            <xs:documentation>List of all message types</xs:documentation>
        </xs:annotation>
        <xs:restriction base="xs:string">
            <xs:enumeration value="ACCESS_TRACKING">
                <xs:annotation>
                    <xs:documentation>See SMT type for annotation.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
...

Running this code:

XmlSchemaSet schemas = new XmlSchemaSet 
{
    XmlResolver = new XmlUrlResolver()
};

schemas.Add("https://www.something.something.com/something", "example.xsd")

results in an error:

The 'http://www.w3.org/2001/XMLSchema:override' element is not supported in this context

Does anyone know if a xs:override directive is supported and what needs to be done to enable that support? Looking through the documentation I have not found anything.