FORTH is a wonderful example of a language that is extensible. You're provided with a number of control structures, but if they don't work for you, you're well within your rights to invent your own. There's a somewhat standard way to break into assembly in FORTH that involves loading the assembler module for whatever processor you're interested in. It adds keyword that generate opcodes and in the process modifies the parser.
So when you talk about extensible languages, I wonder if what you're talking about isn't simply a language where the parser is configurable..