The symfony classes contain sfMixer hooks that give them the ability to be modified at the application level. The mixins mechanism allows multiple inheritance and class overriding at runtime even if the PHP limitations forbid it. So you can easily extend the symfony features, even if you have to modify the core classes for that--the factories configuration is here for that.
Many such extensions already exist; they are packaged as plug-ins, to be easily installed, upgraded, and uninstalled through the symfony command line. Creating a plug-in is as easy as creating a PEAR package, and provides reusability across applications.
The symfony wiki contains many plug-ins, and you can even add your own. So now that you know how to do it, we hope that you will enhance the symfony core with a lot of useful extensions!