You will need a inverter "step-up" circuit, there have been several posted here on CPF
There are several companies that make chips just to do this purpose, you will need to add a inductor, recovery-diode and a cap
The problem is the eff, most of them are less than 60% eff; which means you waste 40% of the energy of the batteries in the conversion, its much more eff to go more batteries and higher voltage