Hello Davidra,
Yes.
However, you will have to settle for a beam that is not as bright, or white.
If all your cells test out to 2000 mAh, the run times will be very close to the same. If you have 2400 mAh Li-Ion cells and 2700 mAh NiMh cells, the NiMh cells (which actually test out to around 2500 mAh) will last slightly longer, but the voltage sag using these cells is higher than that when using the Li-Ion cells.
Now, if you could figure out a way to use Sub C NiMh cells, they would give you around twice, or more, the runtime.
Tom