It can be done by adding custom commands to the appropriate command provider. You can probably get started on the right track by looking at this blog post, the same principles apply: http://world.episerver.com/blogs/Tahir-Naveed/Dates/2015/5/adding-menu-item-to-page-tree-context-menu/
Check out the comments on that blogpost, since Ben from Episerver added his example that I later used for "Open in new tab" functionality.
Anyone have a clue how to extend the contextmenu that appear for content in a contentarea?
Just like there are Displayoptions, i would like to be able to add other tags to the content.