As I understood, you want just copy all the attributes along with the element "SubscriberRequest" into the output tree.
You can write:
Code:
...
<xsl:template match="SubscriberRequest">
<xsl:copy>
<xsl:copy-of select="@*"/>
<DataKey>
</DataKey>
</xsl:copy>
</xsl:template>
...
And also I think it'll be better to think of XSLT transformation as a process which deals with tree(s) and which outputs tree(s). The serialization (XML tree -> .xml file, for example) is a separate process, and a transformation souldn't be viewed as outputting the tag "<x>", followed by some text and then the closing tag "</x>".
Regards,
Armen