I have an existing complex application in Swing which has grown over time . Is it possible to introduce Griffon into such an existing application and convert parts of it to use this framework until I have the complete application converted over? Or Would it make sense to Griffon on a new project?
Given that Griffon controls the application's lifecycle it would be better if you start with a Griffon application and add your "legacy" bits one by one.
If your views use NetBeans Matisse then you can "import" them by calling 'griffon generate-view-script <classname>'. It is very likely that most of the code that you have right now in that application can be placed inside src/main, leaving you the task of wiring up views and controllers. Controllers may be very thin and delegate their job to the legacy code. that is, until you think it's good to move the code to the controllers.
There's also the option of wrapping business code in services. Griffon 0.3-BETA-1 introduced the concept of services as Grails understands it (except they are not transaction by default as no persistence layer is available by default). See the released notes (http://griffon.codehaus.org/Griffon+0.3-BETA-1) for a list of features to come in the next release.