Unfortunately, wordpress is not a good candidate for a PBI since it is not a "stand-alone" program. Wordpress is basically a server addition, meaning that it has to get "installed" into your desired server, and doesn't really work on its own.
I would recommend creating a jail and installing apache or some other server first, then doing "pkg_add wordpress" instead. That way it is more secure from being in a jail, as well as not changing any of your current system packages.
~ Ken Moore ~