Extending functionality of existing (built-in) form elements

Hi,

I am fairly new to Episerver and to this community (this is in fact my first post). :)

I want to extend existing form field (Rich text (with placeholders)), so that the editor can choose (checkbox) if she wants to save the contents of it on form submission.

I know I can create a custom form element with this functionality, but wondering what would be the approach if I want to extend functionalities of built-in form elements. Is something like this plausible?

Thanks a lot!