Is this what you want?
<URL>
<xsl:attribute name="path">
<xsl:value-of select="URL"/>ABC<xsl:value-of select="PARAM"/>
</xsl:attribute>
</URL>
Or to save your fingers, use Attribute Value Templates (avt's) which
basically replace using <xsl:value-of select=".."/> in attributes.
Use:
<URL path="{URL}ABC{PARAM}"/>
cheers
andrew
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list