I'm sure this is a very straightforward task, but I'm missing something
fundamental about the identity transformation.

I want to copy an xml file exactly except if nodeX lacks a childY write
element Y.

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

  <xsl:template match="//snotes:SourceNote[not(snotes:sourceDate)]">
      <xsl:element name="sourceDate"><xsl:text>0000-00-00</xsl:text></xsl:element>
  </xsl:template>

IE: copy all elements complete, except where //snotes:SourceNote has no
snotes:sourceDate child. In that case, write a snotes:sourceDate element
with the value 0000-00-00.