Can I increase the no of operating hours of a battery without increasing its capacity?
12V,1Ah means you've got 12Wh.
14V,1Ah will get you 14Wh.

So yes - you've got more "power" (and you did increase the capacity of the batteries by that...)

Now it depends on how your circuit will use that energy. When it needs less than 0.1Ah @ 14V, it will work longer. If it "burns" the excess voltage, and still need 0.1Ah, regardless of whether it is running at 8,12 or 14V, it will run for the same duration