class javax.enterprise.inject.spi.Interceptor has the following JavaDoc:

Represents an enabled {@linkplain javax.interceptor interceptor}.

The JavaDoc and http://docs.jboss.org/cdi/spec/1.0/html/spi.html#abd SPI Doc seems to imply that you can do the following in a Extension:

public void enableInterceptor(@Observes AfterBeanDiscovery beans) {
  beans.addBean(MyInterceptorType())
}

While you can add them, they are still not enabled, which can only be done via beans.xml in CDI 1.0.