This XSLT adds a level attribute to the c03 components lacking one, in an EAD stylesheet

add_level.xsl

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
      xmlns:xs="http://www.w3.org/2001/XMLSchema"
      exclude-result-prefixes="xs"
      version="2.0">



<!-- IdentityTransform -->
<xsl:template match="/ | @* | node()">
         <xsl:copy>
<xsl:apply-templates select="@* | node()" />
</xsl:copy>
</xsl:template>


  
  <xsl:template match="c03[not(@level)]">
    <c03 level="file">
      <xsl:apply-templates select="@*|comment()"/>
    </c03>
  </xsl:template>
  
  
  </xsl:stylesheet>