Your solution could look like this:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:edi="www.edifabric.com/x12">
<xsl:template match="edi:D_744_1">
<xsl:element name="testfield">
<xsl:copy-of select="." />
</xsl:element>
</xsl:template>
</xsl:stylesheet>
If you have a default namespace in your xml, you have to define it in the xsl as well. Find some more information e.g. here.