If you're using NiMH then you're only getting 1.2v out of each cell, correct?
I tried this on my 5000d but it would only give me a flashing charge LED and eventually led to its bricked state (not directly because it fried the unit, but because it seemed to confuse the internal charging system to the point where the battery wouldn't charge over 40% [perhaps that was due to age, but it wasn't an issue prior to that] which then led me to muck around in the Diag Menu and to do things in there that I shouldn't have).
If you're using 4 - 1.2v NiMH cells on your 6000, then I might try it again and hope for the best.
Did you incorporate any other components in your homemade extender? e.g. resistor, diode?
With the 9v charging thing, a 9v is basically 6 small 1.5v in parallel, so I can't think of how it might work as advertised. Regardless, you're only working with a few hundred mAh.