There are two keys installed to Nokia Symbian devices that are trusted in software installation: Nokia and Nokia OK. The Nokia signature is used in signing the Nokia branded application installation packages and Nokia OK key is used in signing the installation packages which have passed the Nokia OK process. For more information concerning Nokia OK process, go to http://www.forum.nokia.com/ok.
It is possible to load a new root certificate into a Nokia Symbian device. However this key will not be trusted by the device unless the trust settings are not defined properly. This can be done for example from the certificate manager application.