• Pack your com.stackoverflow.${whatever} clases on a jar called stackoverflow.jar
• Create folder com/stackoverflow/main on ${WILDFLY_HOME}/modules
• Put the jar on the last dir
• Create a file called module.xml with the following content:

<?xml version="1.0" encoding="UTF-8"?>
<module xmlns="urn:jboss:module:1.1" name="com.stackoverflow">
    <resources>
        <resource-root path="stackoverflow.jar"/>
    </resources>
</module>

And now you have you module available to applications

For some reason you develop a new version of com.stackoverflow (say version 1.1), then you should:

• Create folder com/stackoverflow/1.1 on ${WILDFLY_HOME}/modules
• Put the new jar on last dir
• Create file module.xml on last dir with the following content

<?xml version="1.0" encoding="UTF-8"?>
<module xmlns="urn:jboss:module:1.1" name="com.stackoverflow" slot="1.1">
    <resources>
        <resource-root path="stackoverflow.jar"/>
    </resources>
</module>

Note the slot="1.1" modification on this example

By default modules are placed on ${WILDFLY}/modules directory but you can have more directories with modules, just edit your standalone.conf (or standalone.conf.bat if you are on Microsoft Windows) and properly set the variable JBOSS_MODULEPATH

For example in Unix/Linux/MacOSX:

JBOSS_MODULEPATH="$JBOSS_HOME/modules:$JBOSS_HOME/myownmodules1:$JBOSS_HOME/myownmodules2"

Or in Windows:

set  "JBOSS_MODULEPATH=%JBOSS_HOME%\modules;%JBOSS_HOME%\myownmodules1;%JBOSS_HOME%\myownmodules2"