> Is it possible to modify one XSLT stylesheet (A) with another 
> (B) to create a new one (C), I guess it is! 

Yes, it is possible.

> Would be great if 
> someone of you could help me, a tiny example would be more 
> than great. 

Here's a tiny example:

<xsl:stylesheet...

<xsl:template match="*">
  <xsl:copy>
  <xsl:copy-of select="@*"/>
  <xsl:apply-templates/>
  </xsl:copy>
</xsl:template>

<xsl:template match="xsl:template[@match]">
  <xsl:copy>
  <xsl:attribute name="priority" select="10">
  <xsl:copy-of select="@*"/>
  <xsl:apply-templates/>
  </xsl:copy> 
</xsl:template>

</xsl:stylesheet>

This adds a priority="10" attribute to any xsl:template with a match
attribute that does not already have a priority.

Michael Kay
http://www.saxonica.com/