Hey Grant,
There isn’t a way to add it within our plugin, however you could do it using a plugin such as Advanced Custom Fields.
However, if you’re planning on using it along with Filter Bar, it may be easier to try to manage that custom content via a taxonomy. From there you could take a look at how the filters are currently registered for event category within the current Filter Bar files.
As a warning though, this could prove to be a little bit difficult. With the other filters, we’re filtering events based on information directly associated with the event. If you’re adding the info to the Organizer, you’ll be trying to filter events on meta assigned to meta (like the movie Inception! 🙂 ).
I’m sorry I wasn’t able to give you a direct answer on this one but hopefully it helps to get you pointed in the right direction.
Thanks!