append string to extracted NameID

[William Lee]

Hello,
Our Shibboleth SP needs to work with partner xyz who puts their username as NameID in their IdP's SAML response message, due to potential conflict of username, I want to append a string "@partnerxyz" to their username, is below the proper way and syntax?
In SP config, AttributeExtractor points to a XYZ-attribute-map.xml which contains:
    <Attribute name="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified" id="username">
        <AttributeDecoder xsi:type="NameIDAttributeDecoder" formatter="$Name!!@partnerxyz" />
    </Attribute>
Main question is whether the part of "!!@partnerxyz" would append the string, maybe just a single "!"? 

Here is an example of their NameID:
        <saml:Subject>
            <saml:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified">usernameabc</saml:NameID>
            ...
        </saml:Subject>


Thanks,William
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