The org.eclipse.wst.xml.core.catalogContributions
extension point lets you add to the catalog (this requires that you have the Web Tools (WST) component of Eclipse installed).
For example the org.eclipse.wst.xsd.core
plugin adds this:
<extension
point="org.eclipse.wst.xml.core.catalogContributions">
<catalogContribution id="default">
<uri
name="http://www.w3.org/2001/XMLSchema"
uri="platform:/plugin/org.eclipse.xsd/cache/www.w3.org/2001/XMLSchema.xsd" />
<system
systemId="http://www.w3.org/2001/xml.xsd"
uri="platform:/plugin/org.eclipse.xsd/cache/www.w3.org/2001/xml.xsd"/>
</catalogContribution>
</extension>