WSDL has a section call <wsdl:types>that you can use for your XML schema:
<?xml version="1.0" encoding="utf-8"?>
<wsdl:definitions
....
xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/">
<wsdl:types>
<xsd:schema targetNamespace="......">
[here you can either import an XSD from a external file,
or add your XML schema directly]
</xsd:schema>
</wsdl:types>
There are at least three WCF extensions that I know will do it for you: