Addition of custom fields per event to Joomla extension Event booking

This concerns the an already modified version of the Joomla extension "Event booking" ([login to view URL]) I need to be able to set the standard custom fields per category (from back-end, so that the front-end user cannot change them) as well as a custom field that the users can add per event. I had thought of the following: addition of a menu to the front-end that allows to create custom fields per events (would look like the custom field menu in back-end and have the same functionalities), just like it has been done for the location creation. The custom fields would be created there, as well as unpublished and published there (the fields which are in the questionaire for custom fields creation would need to be CSS hidable so that I can customize it using CSS override). On front/end under the basic tab of the event submission menu, just before the short description would be a row with a drop/down list with the custom fields that the user already created (and that are published). They would be responsible themselves for keeping it up to date with publishing and unpublishing the custom fields in the custom fields menu. Thanks Tim PS: Maybe make this into a plugin so that it would not affect the main branch of the event booking system (ie I would still be able to get updates for the system without having to maintain a modification.)