how can i use an external light in my camcorder JVC GR-D70U

Not sure what you mean here. An external light is a completly independent piece of kit to any camera. After all I would describe the Sun as an external light source wouldn't you?

Is this more a question of fitting it to the camera or something? The only camcorder light I have fits with a hot-shoe fitting (A Pag C6). Does the camera have one of those? If not then you will need to visit you local camera shot and get some sort of bracket that you can connect the camera to (as though it were on a tripod) that has an extra hot shoe adaptor.

Something like this? http://tinyurl.com/yht3gz (short version of amazon link)

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