Hi,
Thanks for using our plugins. I can help out here.
It is true the additional fields do not have a visual editor.
Although possible to edit the additional fields output it would be a big customization to include a visual editor in the admin.
Another option could be to use a plugin like the Free Version of Advanced Custom Fields to add a Visual Editor Field to the events admin.
Then following our themer’s guide you can edit the single event template in your child theme and place the content of the field where you like it:
https://theeventscalendar.com/knowledgebase/themers-guide/
I can help answer some more question to get you started if you like, but unable to provide all the coding for a customization.
Let me know.
Thanks