inserting html page block on page not listing on page builder

Is there a way to insert a HTML page block within a page that you cannot access in admin/pagebuilder?

For example, I would like to insert a specific block on 

http://www.mysite.com/forum/#group/General.htm

This is the page for a specific forum group but is not available in page builder so will not show up on any page block edit tools...

i don't think its possible to add a block in a html page other than dolphin pages.