Years ago, I had a Vivitar extender that was coupled with a variable extension tube - it was quite useful for macro. I suppose you could say it allowed independent control of magnification.
That's different than what OP was referring to. An extension tube is, as you say, is good for macro photography. Not so good for increasing the focal length which is what OP was talking about. A variable extension tube is pretty simple, since extension tubes are basically just empty space (such as bellows), whereas tele-converters usually contain 1 or more lens elements to re-focus the image to get it to hit the focal plane correctly.