Is it possible to use a Symfony plugin in a CMS being built with another framework like Cake or Zend? How independent are those plugins?
It depends on the plugins and the frameworks. But in all likelihood you will have to rewrite at least part of it.
question is a little too abstract.
the answer to "is it possible...?" is "yes" but the needless complexity associated with mixing frameworks of different design intent is questionable at best.
cake / zend are loosely-coupled intent fw, i.e. you can pick and choose script from various places into your cms project-set and it will still work. but symfony is a tightly-coupled fw where placement of scripts are expected in "sf" standard places or things break.
each on its own has nice advantages, but if study google results for "symfony sucks", "cakephp sucks", and "zend framework sucks" you can just imagine combining the disadvantages of each fw to ensure a no-sleep-nights worst-case scenarios. :D
2c
Probably better to see how the plugin works and refactor it for the framework of your cjoice.