Customising the text block in exhibits

I want users to be able to embed content from an URL (in exhibits text blocks) such as images, audio and documents like it is done with the Insert->Media option which is by showing a user-friendly form with certain input parameters (URL, width, length…). All that without having to modify the HTML source code since most of our editors don’t know how.
I was wondering where can I modify the code in order to do this or if there exists a plugin that would do the job. (Looking in ~/plugins/ExhibitBuilder/views/admin/exhibits/block-form.php but don’t know if is the right place)

Found out the right path: /admin/themes/default/javascripts/globals.js and then there I can hack it a little bit with some TinyMCE plugins (in the initParams object). For instance: