maven: How to add resources which are generated after compilation phase

I would suggest to define the output directory for the XSD files into target/classes (may be with a supplemental sub folder which will be packaged later during the package phase into the jar. This can be achieved by using the maven-resources-plugin.

<project>
  ...
  <build>
    <plugins>
      <plugin>
        <artifactId>maven-resources-plugin</artifactId>
        <version>3.0.2</version>
        <executions>
          <execution>
            <id>copy-resources</id>
            <phase>process-classes</phase>
            <goals>
              <goal>copy-resources</goal>
            </goals>
            <configuration>
              <outputDirectory>${project.build.outputDirectory}</outputDirectory>
              <resources>          
                <resource>
                  <directory>${basedir}/target/xsd-out</directory>
                  <filtering>false</filtering>
                </resource>
              </resources>              
            </configuration>            
          </execution>
        </executions>
      </plugin>
    </plugins>
    ...
  </build>
  ...
</project>

You need to take care that the resources plugin is positioned after the plugin which is used to call the wsgen part. You can also use the prepare-package phase instead to make sure the resources will be correctly packaged.