Constellio offers three solutions for extensions:

Connectors (Google Enterprises Connectors):

The connectors allow Constellio to extract and index the content of various systems in your organization. Constellio is based on the connector architecture of Google Search Appliance, which makes all the connectors compatible with both solutions
http://code.google.com/intl/fr/apis/searchappliance/documentation/connec...

Plugins:

Constellio offers a simple plug-in feature to extend its behavior and add new features.
http://code.google.com/p/jspf/

Google One Box:

Google's technology for integrating external search engines.
http://code.google.com/intl/fr/apis/searchappliance/documentation/64/one...