You can do this at the same time and as many times as you want:

<xs:complexType name="Person">
   <xs:sequence>
   <xs:element ref="my:name"/>
   <xs:element ref="my:major"/>
   </xs:sequence>
</xs:complexType>

<xs:complexType name="ExtendedPerson">
   <xs:complexContent>
      <xs:extension base="my:Person">
         <xs:element ref="my:address"/>
         <xs:element ref="my:extras"/>
         <xs:attribute name="grade" type="xs:string"/>
         <xs:attribute name="level" type="xs:string"/>
         <xs:attribute name="birth-date" type="xs:date"/>
      </xs:extension>
   </xs:complexContent>
</xs:complexType>