Writing a plugin is not easy.
@Joshua Parker Ruehlig is one of the few to do it. If all you are trying to do is get rtorrent working, the easiest thing is to just create a new jail, and install rtorrent there. Look for any guide on installing rtorrent in FreeBSD, and follow that in the jail (for more information on working with jails, check out the FreeNAS or FreeBSD documentation).
Other than that, FreeNAS 10 is going to do away with plugins, so there's not much of a reason to develop a new plugin at this point in time. FreeNAS 10 is going to support docker containers. FreeNAS 9.10 has the necessary under-the-hood support to do what FreeNAS 10 will, but does not yet have a GUI feature for working with Docker containers. If you are so inclined, you could get an rtorrent container working with a little tinkering. There are some posts in the FreeNAS 10 development section that talk more about that feature, and some others around here have done just that in FreeNAS 9.10.