how do i overide sitemap to a .html

hi im trying to point lenya to look at a html page when it receives a 
request for portfolio.html

this is the code im using on the default sitemap.xmap, copied to my 
publication folder

<map:match pattern="portfolio.html">
<map:read src="content/ovr/portfolio.html" mime-type="file"/>
</map:match>

my question is, where do I insert the code. The xmap is a little 
confusing to say the least

thanks in advance all