Hi Rainer,
This would be somewhat difficult to achieve and I won’t be able to fully help you out with a solution but maybe I can point you in the right direction. First, if you look in /wp-content/plugins/the-events-calendar-community-events/views/event-form.php on line 53, this is where the category list is called. And then if you trace back formCategoryDropdown() you’ll find that in /wp-content/plugins/events-community/lib/tribe-community-events.class.php
Here’s what I think you’ll want to do to get around to customizing the category list. Make a copy of /wp-content/plugins/the-events-calendar-community-events/views/event-form.php and place in an ‘events/community’ folder in your active theme so you can safely make changes to it. Then, what I would do is swap out the formCategoryDropdown() function call with the code used for the whole function that you can find in /wp-content/plugins/events-community/lib/tribe-community-events.class.php – and then you can modify the code however you want to account for sub-categories.
I hope that helps and good luck!
– Jonah