For a file based module, the above method won't work. You instead need to just copy the directory to the module directory. So the target name would instead be:

<target name="helloworld" >
       <copy todir="${deploy.modules.dir}/helloworld">
           <fileset dir="${basedir}/customModules/helloworld">
           </fileset>
       </copy>
   </target>

With potential include or exclude elements inside the fileset node, to filter out particular files.