gistfile1.php

<?php

$root
    ->fixXmlConfig('path')
    ->children()
      ->arrayNode('paths')
          ->beforeNormalization()
              ->ifString()
              ->then(function ($v) {
                  return array($v);
              })
          ->end()
          ->prototype('scalar')->end()
      ->end()
  ->end()
;

gistfile2.yml

# posibilities
paths: ['foo/bar', 'bar/foo']
paths: 'foo/bar'
path: 'foo/bar'

gistfile3.xml

<path>foo/bar</path>
<path>bar/foo</path>