Extension from another namespace

I have a schema proced.xsd that I redefine in this schema:

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:aGivenExtension="http://aGivenExtension"
targetNamespace="http://aGivenExtension"
elementFormDefault="qualified" attributeFormDefault="qualified">

	<xs:redefine schemaLocation="proced.xsd">
		<xs:complexType name="supportEquipDescrElemType">
			<xs:complexContent>
				<xs:extension base="supportEquipDescrElemType">
					<xs:sequence>
						<xs:element ref="aGivenExtension:supportEquipMaterialSetRef"/>
					</xs:sequence>
			
				</xs:extension>
			</xs:complexContent>
		</xs:complexType>
		
	</xs:redefine>	
	
	
	<xs:complexType name="materialSetRefType">
	<xs:sequence/>
		<xs:anyAttribute processContents="lax"/>
	</xs:complexType>
	
	<xs:element name="supportEquipMaterialSetRef"
type="aGivenExtension:materialSetRefType"/>
</xs:schema>

But I get this error in XmlSpy:
"Redefining type definition
'aGivenExtension:supportEquipDescrElemType' must be an extension or
restriction of itself."

which is unclear to me.

I want to redefine the "supportEquipDescrElemType" from proced.xsd.
proced.xsd has no namespace in it.

What is the correct process to extend it with some elements from the
"aGivenExtension:" namespace?