XSD: own element type

[marco]

Hi all,
I have my own XML scheme, (namely, MyScheme.xsd), where various
simpleTypes and complexTypes are listed.

I want to arrange my scheme with the insertion of a element of type
"identifier", returned by an external API.

Something similar to:

<xsd:complexType name="MyComplexType">
<xsd:sequence>
<xsd:element name="Value" type="xsd:string">
<xsd:element name="Id" type="extAPI.identifier">
</xsd:sequence>
</xsd:complexType>

How can it be done?

Thank you very much!