This post will guide you how to extend the TinyMCE editor EPiServer with a custom button and dialog.
Create a visual TinyMCE Plugin – SoftResource.
EPiServer offical doc:
http://world.episerver.com/documentation/Items/Tech-Notes/EPiServer-CMS-6/EPiServer-CMS-6-R2/TinyMCE-Developers-Guide/
Dynamic content in xhtml editor (another type of solution)
http://blog.wsoft.se/2015/03/03/creating-simple-dynamic-content-in-episerver-cms-6-r2-ted-gustaf/