+1 In series the voltage adds, in parallel the capacity adds(mah) for a longer runtime(though with 3 eneloops in parallel the capacity would be 6,000mah vs 10,000mah NiMh D).
Depends on how the adapter is set up. Most are in series, so the module would fry from the voltage. But it should work if the adapter is wired in parallel and series(3 parallel for each D size and 3 of those in series).