Good evening Andy and welcome to the Events Calendar Support forum!
Thank you for reaching out to us with your suggestion.
This is a great idea.
In fact, you can suggest it or cast your vote on other features in our User Voice forum.
is it possible to add an new Admin-Option by a theme function?
Absolutely! To do that, you might want to read our Themer’s guide which details how to go about it.
To make a long story short, any template customizations done in this way will have priority over the plugin’s code.
And, if you don’t mind, you are always welcome to share any snippets you come up with. I am sure other people might be interested in such a feature.
Let me know if that helps.
Have a great day!
Geoff B.