How to Use XSLT Extension Points from a Publishing Template
It is possible to declare a WebHelp XSLT-Import Extension Points in an Oxygen Publishing Template to override or add additional XSLT processing.
Example:
    <publishing-template>
    ...
    <webhelp>
        ...        
        <xslt>
            <extension
              id="com.oxygenxml.webhelp.xsl.createMainPage"
              file="xsl/customMainPage.xsl"/>
        </xslt>