I've never ran across a project that screamed at me so loudly to be put into an OSGi framework.
There's multiple stages to an application that's OSGi based. The lowest level of design is just updating your jar files so that the implement the necessary bundle information. This is pretty straightforward and there are ant tasks that will do this for you, using the bndtools.jar and this would provide a plugin framework OOTB
The second level, and where it gets interesting is the conversion of the application into a services framework. Being able to implement a level of separation of concern is where OSGi shines.