HOW TO Insert an element in WP-toolbar?

Hi, how can insert ‘option’ item in wp-text-toolbar?
for example in wp basic text editor there is a option item ‘Heading 1- Heading 2 – Heading 3 – heading 4 – Paragraph’;
I’d like to enter an option item with more city: ‘Roma – New York – Milano – Paris – London…’ on click insert item in page or article.
Which files should be avoided in order to submit this?, more template insert this option in wp-toolbar.
thank you all for the response!