Event form with a master category

Hello,

I need a event form where user can create an event and this event gets a “special / master” category. The user can set a category on it’s own, but the user’s category where creaete as an subcategory from the master category.

I need this for a convention site where user create there own event on this convention. After this the events where created I which there is an list with all of the events of the any user how create on.

How do I do this?

Regards

ThoGri