Is the event_calendar plugin extensible?

I'm evaluating elgg for a local community project.

It's great, and feels like a good fit to the project already, but we have a couple of requirements, EG a "booking details" link field in the event_calendar plugin. I've crowbarred one in, which was a good experience learning how the plugin is architected, but I'm sure that updates to the plugin would overwrite my changes.Is the event_calendar plugin designed to be extensible in this way (adding custom fields) or, indeed, at all? Or is there a generic way to add custom fields to entities (like a WordPress "custom fields" plug that lets me add custom fields to any kind of post).

Thanks for your help.