Hi, Augustin!
If I understood correctly, you want to extend JBoss HTML Editor with additional Tabs ("Visual/Source - Source - Preview" - are the Tabs (but not "tags") of the editor, and yes the different editors are used inside these Tabs to provide ability to edit/view the file content in some way)...
Currently JBoss HTML Editor cannot be extended in the way you have described. The idea is interresting by itself, but, IMHO, if realized, may cause many issues because we cannot make sure that the editor you want to provide for such extention will not impact the MultiPageEditor functionality and stability. Also, there are toolbars/context menues and many other things that are to be extended when you add some new editor. And while doing such extention you may also face come conflicts.
BTW, what kind of editor you're going to add to the JBoss HTML Editor?
Best Regards,
Victor Rubezhny