Can I use variables in XML files?

You can declare an entity reference for chief and reference it as &chief;:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE foods [
  <!ENTITY chief "CHIEF_NAME!">
  <!-- .... -->
]>
<foods>
  <food>
    <name>French Toast</name>
    <price>$4.50</price>
    <calories>600</calories>
    <chief>&chief;</chief>
  </food>
  <food>
    <name>Homestyle Breakfast</name>
    <price>$6.95</price>
    <calories>950</calories>
    <chief>&chief;</chief>
  </food>
</foods>