How to add an extra source directory for maven to compile and include in the build jar?

You can use the Build Helper Plugin, e.g:

<project>
  ...
  <build>
    <plugins>
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>build-helper-maven-plugin</artifactId>
        <version>3.2.0</version>
        <executions>
          <execution>
            <id>add-source</id>
            <phase>generate-sources</phase>
            <goals>
              <goal>add-source</goal>
            </goals>
            <configuration>
              <sources>
                <source>some directory</source>
                ...
              </sources>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
</project>

NOTE: This solution will just move the java source files to the target/classes directory and will not compile the sources.

---

Update the pom.xml as -

<project>   
 ....
    <build>
      <resources>
        <resource>
          <directory>src/main/config</directory>
        </resource>
      </resources>
     ...
    </build>
...
</project>

http://maven.apache.org/guides/mini/guide-using-one-source-directory.html

<build>
<sourceDirectory>../src/main/java</sourceDirectory>

also see

Maven compile with multiple src directories