Did the Luftwaffe configure the Messerschmitt Bf 109 with an external fuel tank?

I don’t know, there are some references to a “drop tank” which may be internal in a bomb bay, or external.

However, should be covered in this book: http://books.google.com/books?id=nnSbAAAACAAJ&dq=isbn:0760311943&cd=1 if you can find it

They did put drop tanks under some of them, I think. I dont think they used them with the FW 190.

There was an L-shaped on-board main fuel tank under the cockpit floor and behind the main cockpit bulkhead, but this was supplemented by wing-mounted drop tanks. These were often jettisoned in combat situations to improve maneuverability and safety.